If you have access to the DMG (if you're DMing, you're going to want it) page 57 has a chart listing how much XP an encounter of level X for a group of 4, 5, or 6 PCs should be. You ought to be able to modify this for a group of 2 fairly easily by taking the XP for a 4-person encounter of the level you want and dividing by 2.
In case you don't have it handy, here are the XP totals for a group of 2 for the first 10 encounter levels:
- 200xp
- 250xp
- 300xp
- 350xp
- 400xp
- 500xp
- 600xp
- 700xp
- 800xp
- 1000xp
Remember that not every fight should be the same level as the party. Fights of the party's level are average difficulty (or easy if the party is optimized), fights of lower than the party's level are easy (or speedbumps at best if the party is optimized), and fights that are higher than the party's level are hard (how hard again depends on party optimization). Generally any fight of less than party level -2 or -3 is too easy, and any fight of party level +4 or more is too hard for all but heavily optimized parties. The same range also applies when selecting monster levels, so monsters should be within 3 levels of the party; a single regular level 10 monster might be the right amount of XP for a level 1 encounter (in a 5-person party), but its defenses and attack values will be much higher than the party's.
Thus, if you want a typical fight for a level 1 party with 2 PCs, you should use 200xp worth of monsters. If you want a challenging fight for a level 1 party with 2 PCs, you should use 300xp of monsters. You can compare a fight's XP total to the recommended XP for a 5 person to estimate what level the Kobold Hall fights should be, then turn it into an appropriate fight of the same level for a 2-person fight.
That said, paladin and warlord is a relatively good 2-person combination (as long as the paladin has a good MBA), so you might find that you need to add in a little extra XP worth of monsters. If you find that adding monsters makes combat take too long (paladin & warlord combo has a lot of healing, but isn't a damage powerhouse), try going back to the normal number of monsters, but double their damage and cut their HP in half.
The system is designed to accomodate this
...but without the DMG it's a little tricky.
The science:
Basically, each enemy has an XP value. This is how much XP it's worth when it's defeated (divided among those who defeat it), but it's also useful for building encounters.
Here's how you build an encounter in 4e: You take the XP value of a "standard"-type enemy of the same level as the party, and multiply that number by the number of PCs in the party. The result gives you a "budget" that you use to "buy" enemies to create an encounter of average difficulty (the party is unlikely to die, but will expend a noticeable amount of resources --consumables, healing surges, daily powers-- during the fight).
For a more difficult fight, increase the level of the standard-type enemy whose XP you're using as the baseline multiplier to get your budget, up to four or five levels above the party. For an easier fight, drop the level down by three or four. The extreme ends of this will produce boss-level fights, or make-the-players-feel-invincible routs.
The art:
The DMG recommends actually using enemies up to five levels higher than the party for boss fights, but in my experience this is more frustrating than interesting; it's better to use "solo" type monsters of the party's level. The challenge level will be similar but more fun.
Combine standards, elites, and minions for interesting fights. Minions die quicker and elites last longer, so if there's an NPC or ability you want to be present throughout the fight make it a tougher monster type.
Use soldier (defender) and lurker types for longer more drawn-out battles, brutes and strikers for shorter, more intense fights.
If you've got a combination of enemies whose abilities support each other in significant ways, or you're adding strange terrain, remember that this may make the fight harder than its XP budget will imply.
The mechanics of monsters changed partway through 4e's tenure
With the publication of the Monster Manual 3, monsters got their hp reduced, their damage increased, and their powers were made a bit more interesting. This makes fights take a little less time while being a little more tense, but the overall resource drain per fight is pretty much the same. If you can get your hands on them, use post-MM3 monsters whenever possible until you're familiar enough with them to adjust the earlier monsters to fit that ethos. If you can't, don't worry about it too much. You'll learn to fiddle with monsters based on experience, and until then the fights will be a little more tedious than they'd be with MM3 monsters.
To that end, seriously consider a D&D Insider subscription. It provides a searchable compendium of every mechanic --rule, monster, item, class, race, etc-- ever officially published, a solid character builder AND a solid monster builder, and downloadable access to all the Dungeon and Dragon magazines for 4e. AND all the errata are kept up-to-date across the compendium and builders. I was suspicious of the service at first, but quickly found it to be nearly indispensable.
Best Answer
There are two cases here to consider. The first, a character like a fighter that relies on attacks for dealing damage. The second, a caster with access to spells which can force saving throws.
Case 1: Character cannot contribute by forcing saving throws
Take a good ole sword and board fighter. Their primary contribution to combat is being engaged in melee combat, making lots of attacks and taking some hits. 4 levels of exhaustion severely diminishes this character's ability to do anything in combat. They have disadvantage on all attacks, they are slow, and their hit points are halved. This character is unlikely to contribute much to the combat. If the character with exhaustion is someone like this, you might as well not count them in the party size when estimating encounter difficulty. They are likely to make things worse for the party as they try to keep this character alive.
Case 2: Character can contribute by forcing saving throws.
Now, suppose our exhausted character is a wizard or sorcerer who has access to a selection of spells that force saving throws. Their offensive capabilities are largely unhindered by 4 levels of exhaustion. Fireball doesn't care how tired you are, it still goes BANG! Sure, this character is even more squishy and vulnerable than before, but they are still in a position to contribute, even in an exhausted state, and they can do so while staying out of the fray. The fighter has to be in melee range to even have a chance of doing anything.
Conclusion: determine if the character can realistically contribute, if not, don't count them when considering difficulty.
There is obviously some in-between with these two cases, so you'll have to figure out where the character falls here. If they can still contribute in some meaningful way, you can just estimate that combat will be a little harder than expected. If the exhaustion severely hinders their ability to contribute, estimate difficulty by reducing the count of the party by 1.
If you're worried about a particular encounter being too hard, make it a two-stage encounter and adjust stage 2 as needed.
Every DM knows that encounter building is tough. Estimating encounter difficulty is far from being a well behaved science. When I am going into an encounter and I am concerned about possibly killing the whole party, I make the encounter a 2 (or more) stage encounter, so I can adjust later stages on the fly as needed. Essentially, I design an encounter that should be somewhat easy, and then I design a second part to that encounter with a range of possibilities. If part one is going great, I can drop the harder version of part two. If part one is going really bad, I can drop the easy version of part two, or get rid of it altogether. This allows you to get a real time sample of the party's performance so you can respond appropriately, hopefully resulting in an appropriately challenging encounter that doesn't kill everyone.