There are a few threads floating around, but imo there are 2 ways to go with this (speaking of PVE now):

a) do a healer+tank combo and try and set it up so you can easily add 3 dps/cc and do instances, wont level any different than 1 toon, but will have easier time to LFG (hopefully)

b) do 2x dps to be able to burts down quests quicker, which I guess will help in pvp to

Either way you wont solo instances but with both combos you will be able to do harder quests and quest quicker than solo, but you will loose some time managing 2 boxes in terms of questing etc.

My experience of 2 boxing have strictly been playing a mage/warlock/hunter with my paladin following as a heal bot to ease doing dailies for the pally.