The Dungeon Master's Guide on Poison says, "One dose of poison smeared on a weapon… affects just a single target" (296). However, the game is silent on what happens when lone dose of contact or—important here—injury poison strikes multiple creatures simultaneously. It's possible the DM will rule that no creature is affected by the poison—saying it's too diffuse or whatever—, or the DM could rule that but one (possibly randomly determined) creature is affected from among those who could be affected, or a DM could rule simply that all creatures suffer the poison's effect.

If it's the first two rulings—or, honestly, if it's almost any other ruling except the last and the effect doesn't explicitly allow multiple creatures affected by a lone dose like this fine answer's suggestion of the magic weapon special ability toxic (Drow of the Underdark 96) (+1 bonus; 0 lbs.)—then you'll be largely out of luck, stuck with this answer's final entry.

Apply a dose of injury poison to a weapon or to ammunition that affects an area

If the DM allows a contact or injury poison to affect all creature hit by a weapon if that weapon hits multiple creatures simultaneously, then a poison-using warrior needs one of the rare weapons or ammunition that hit multiple foes simultaneously. Below are three mundane weapons that can do just that. Caution: Some options below may reduce the campaign's verisimilitude.

…Like a caber

The caber (Masters of the Wild 26) (10 gp; 100 lbs.), an exotic weapon, was never updated by the 3.5 revision therefore making it subject to minor adjustments by the DM.

^{A creature that possesses the Drow of the Underdark feat Master of Poisons (51) can take a swift action to coat with poison a caber. That's how you know this is a fantasy game. That and the existence of highlander drow.}

Despite raising a few uncomfortable questions about its interaction with certain magic weapon special abilities, the caber remains relatively playable as originally written. Its description says

A caber is a heavy pole that you can throw at one or more targets grouped closely together. To throw a caber, you must target a 10-toot-square area and hit AC 15. Success means that everyone in the target area must make a Reflex save (DC = your attack roll) or move 5 feet backward. If a creature or object in the target area is incapable of movement, it takes 2d6 points of damage. The caber is normally used for breaking up military formations. (ibid.)

The caber's stat line has it dealing bludgeoning damage (and although its damage entry is — it still has a ×2 critical hit multiplier), and it has a range increment of 10 ft. (Plus no one is one-hit sundering it: a caber has hardness 5 and 120 hp!) Thus, according to this description, so long as all the foes in that 10-ft.×10-ft. space can't move—like if they're paralyzed, sleeping, or just boxed in by your allies—, they're dealt the caber's damage, and that should trigger the injury poison that you put on a log you just tossed at them. Well, the space they're in, anyway. (Some might call poisoning a caber overkill; others will call this winning.)

…Or a unit of sand blaster ammunition

Buried in the description of the sand giant is the sand blaster (Monster Manual III 58) (30 gp; 0 lbs.), an exotic weapon.

^{This unit of sand blaster ammunition is coated with one dose of injury poison and it makes a mess; this makes it perfect for—and against—goblins and kobolds, for instance.}

The sand blaster's description says

A sand blaster is a Large exotic ranged weapon made from long tubes. It creates a 10-foot cone of sand, doing 1d8 points of damage (Reflex DC 22 half). Living creatures that fail their saves are tormented by itching skin and burning eyes, imposing a –4 penalty to AC and a –2 penalty on attack rolls for 3 rounds. The save DC is Constitution-based. (It relies on the user’s ability to blow a hearty gust of air through the tube.) A sand blaster uses 5 pounds of sand as ammunition, and packing a sand blaster with one charge of ammunition is a full-round action.… (ibid.)

This weapon is particularly interesting because—as can be seen from the description—it doesn't mandate an attack roll. This makes taking the feat Exotic Weapon Proficiency (Player's Handbook 94) for the sand blaster (possibly?) pointless. Further, its description specifically says that the 5 lbs. of sand is one charge of ammunition, making that pile of sand the user scooped up just about perfect for kneading the injury poison into. So what should happen is that a sand blaster user scoops up 5 lbs. of sand, somehow coats it with the injury poison, loads the sand blaster with that 5-lb. scoop of sand that's one unit of sand blaster ammunition, and blows that unit of ammunition from on the sandblaster into a 10-ft. cone; this forces creatures in the 10-ft. cone make Reflex saving throws to avoid damage from the ammunition, and failure indicates that the creature makes a Fortitude saving throw against the poison.

What's less clear is how reducing its size affects a sand blaster: it has no stat line therefore no Damage entry therefore the DM must determine if the a sand blaster sized for a Medium creature deals 2d6 points of damage, deals its damage in a 5-ft. cone, both, or even just does nothing, the DM ruling (harshly, in this reader's opinion) that a sand blaster must be sized for a Large or bigger creature for it to do anything but annoy folks.

Further, because, "most projectile weapons require two hands to use" (113), and the sand blaster description doesn't have rules for 1-handed use like, for example, the light crossbow (115–6), the DM may rule that—perhaps in addition to ruling that a sand blaster sized for Medium creature is ineffective —, a sand blaster sized for a Large or bigger creature simply can't be used by a Medium creature because of the effort necessary to employ it (i.e. a Large 2-handed weapon is simply too big for the typical Medium creature to wield). Certainly, this won't be the last time: Ask the DM.

…Or a Large razor net

The Large net (Dungeon #99 35) (40 gp; 20 lbs.) costs twice as much and inexplicably weighs 3⅓× as much as the typical net (PH 117, 119-20) (20 gp; 6 lbs.). Further, there also exists—in a Wizards of the Coast-licensed product published by Paizo—the razor net (Dragon Compendium Volume 1 112, 115) (50 gp; 15 lbs.). Thus, by extension, a creature should be able to acquire—with the DM's permission, of course—a Large razor net that combines the Large and razor nets for a 100 gp if the user's willing to tote around 49 lbs. of netty goodness. (Hey, fractions round down—take that, ½ lb.!

The typical net is a 1-handed weapon according to the special attack Two-weapon Fighting (PH 160). The Large net description says that it can be wielded as a two-handed weapon by a Medium creature that possesses a Strength score of at least 20 and that the Large net can entangle up to 2 Medium creatures that are adjacent to each other (or a proportionate number of Large, Small, or Tiny creatures). The razor net is disposable—eventually those trying to free themselves from it will succeed and the razor net's barbs invariably befoul the razor net itself, both effects ultimately ruining the razor net, making each a single-use item—, but a razor net deals any creature that attempts to escape it 1d6 points of slashing damage automatically. (And, for reasons this reader can't discern, it has a ×2 critical multiplier.) Hence a Large razor net that's coated with poison will poison those it entangles if they attempt to escape it by bursting it or by making an Escape Artist skill check.

Again, ask the DM's permission for a Large razor net. It's technically off-the-books twice: first, the razor net is from a somewhat dubious source, and, second, the Large razor net is a theoretical extrapolation of another weapon that most folks don't even know about. (Although it does make sense that, like, giants would use bigger nets!)

Use a magic weapon special ability or a spell to duplicate an already poisoned unit of ammunition or weapon

One can ask the DM about the magic weapon special ability splitting (Champions of Ruin 42) (+3 bonus; 0 lbs.), the 3rd-level ranger spell arrowsplit [trans] (CR 28), and the 3rd-level Sor/Wiz spell threesteel [trans] (Dragons of Faerûn 119). All three effects magically duplicate either a weapon or ammunition, but all three are silent if any effects present on the original are likewise present on the duplicate, be they magical effects (like a spell) or mundane effects (like most poisons).

Turn a poisoned melee touch spell into a ray then into a cone or a burst

The general feat Poison Spell (DrU 51) only works on spells with an entry of Range: Touch. A spell affected by the feat Reach Spell (Complete Divine 84)—with its metamagic adjustment of +2—"effectively becomes a ray" if it before had the entry Range: Touch. Once that metamagic feat's applied, then it's just a matter of also employing either of the following metamagic feats:

• The feat Ray Coning (Dragon Annual #5 26) with its metamagic adjustment of +2 causes a ray spell to become a 30-ft. cone.
• The feat Ray Burst (Dragon Annual #5 26) with its metamagic adjustment of +3 causes a ray spell to become a 10-ft. burst centered on the caster.

Not inconsequentially, both the Ray feats above essentially add to the affected spell the entry Saving Throw: Reflex negates, which is another saving throw to beat to poison a multitude of enemies. A DM could still nix this idea, though, by ruling that because it's a general feat rather than a metamagic feat, the premise of the feat Poison Spell must be adhered to. That is, a spell that gains a range entry other than Range: Touch simply can't be affected by the feat Poison Spell.

However, seeing as how even the 0-level spell touch of fatigue [necro] (PH 294) modified in this way becomes a 4th-level or 5th-level spell that also expends a dose of injury poison and that mandates failing four saving throws for the entire combination to have its full efficacy (a Ref save to negate the spell, a Fort save to negate the spell again, then a Fort save now and 1 min. later to negate the poison)—and that's ignoring the three-feats entry fee!—, this DM would struggle to disallow the combination for balance reasons except in extreme circumstances. (One circumstance could be, for example, largely free access to way too much of an obscure deadly poison through the feat Hidden Talent (Expanded Psionics Handbook 67) and picking the 1st-level shaper power psionic minor creation [metacreativity] (Expanded Psionics Handbook 121), but even then with that process there are issues.)

Use feats to spit in a 15-ft. cone an injury poison that's linked to a poisonous bite

This, I'm certain, isn't what the question's asking for, but I'm including it for completeness. There are several methods a creature can use to get a poisonous bite attack: the yuan-ti graft poison fangs (154) (8,000 gp; graft); a totemist shapes to her shoulders chakra and binds to her totem chakra the soulmeld phase cloak (Magic of Incarnum 82); a creature spends a daily use of the supernatural ability wild shape gains for 1 min. a venomous bite attack using the wild feat Serpent's Venom (Complete Divine 84). Finding the optimal poisonous bite for a specific campaign is left to the reader.

Once a poisonous bite is gained, though, the creature can take the Serpent Kingdoms monstrous feat Spit Venom (147), and the feat's benefit allows the creature to spit its poison as a ranged touch attack at a foe up to 30 ft. away, success causing the poison to affect the target as if it had been delivered via the creature's bite. More importantly, that feat is the prerequisite for the monstrous feat Deadly Spittle, its benefit saying, "You can spray your spit in a 15-foot cone-shaped burst" (145).

Unless the creature possesses multiple poisonous bites—certainly not, by any means, an impossible situation—, this combination of feats will typically only allow the creature to affect multiple foes with one specific injury poison. (Although absent from the question, this reader infers that one appeal of poison is its versatility!)

Note: I'd be remiss were I not to mention the 4th-level druid and 5th-level cleric spell spit poison from Dragon #304 (37-8) that allows the caster to spit at a foe as a ranged touch attack any number of poisons that currently affect the caster. While it'll only work on contact poison absent a poison bite—and the caster must also first avoid dying from whatever poisons course through the caster's veins—, the effect is still pretty cool, and one of the few ways to affect one creature with multiple doses of a poison simultaneously.