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
Don't
I would suggest to you to just accept the new character without a lot of fuss. The dead character is not really related story-wise to the new character (except in special cases). The thing that connects them is that they are played by the same player, a metagaming connection, not an in-game one.
Therefore, I feel that making a player jump through hoops so he can continue to have fun with his friends does not seem to be the optimal way to increase enjoyment for everyone.
A bit of background
In the past, I put too much time and work into making new characters fit into the story
My campaigns are usually quite deadly, so people dying is a regular occurrence. When I started out as a DM, I put quite a bit of time into this, making sure each characters backstory is deeply integrated into the campaign plot, etc. These days, I rather look forward than back:
I stopped doing that. New characters just join the party. Why?
The basic issue is:
What do you want to spend your time on?
I have found that, at least in my group, it's more fun for everyone to just continue playing and letting the character grow into the campaign more naturally. Instead of preparing and investing lots of time into the backstory, I'd rather invest the same time into hooks and options for the new character to be fleshed out in play, to do the integration at the table.
The final point:
The meaningfulness of the character death lies in the story, in-game
You are free to flesh this out as you want. But it's not related to the new character at all. And some character deaths are meaningless. A fluke, a series of mistakes, whatever. Not every death can be a heroic sacrifice to save the world.
Since the connection between the death of the old character and the appearance of the new character is a metagaming one, I think that the new character does not and cannot really affect the meaningfulness of the death.