If you're taking suggestions:
This is not a party, it is a hero and his sidekick. Play it like that! Whenever heroic stuff is called for (i.e. encounters of ECL >6, which will get the sidekick straight-up killed), the hero should do it alone. Try and separate the two more or less often, to present them with challenges appropriate to the characters they're playing - with only 2 PCs, this is easy to do.
There could still be times when the hero is up against an overwhelming challenge (say, a Fire Giant Fighter 6), and the sidekick must save the day (say, by fighting/tricking the Fire Giant's Azer minions and freeing the imprisoned hero).
At other times, the sidekick micht become a liability, and much of an encounter's difficulty would stem from the fact that the hero must keep him alive. For example, the 10th level PC could easily take on a tribe of Orcs all by himself - but keeping his sidekick alive takes a little more effort.
To sum up: play to the strengths of your 'party' setup, but try never to just give them a straight-up fight at an appropriate ECL and expect both to come out alive. Also, don't just give Mr. Lvl2 XP by the table for defeating it, or he'll catch up ridiculously fast.
If you want him to catch up, though, just let him start at a higher level already! D&D's XP and leveling system doesn't well support mismatched 'parties'.