When 2-boxing:

Exp/killing = 50%*2 chars
Exp/hour on gathering-quests = 50%*2 chars
Exp/killing-quests = 100%*2 chars
DPS = ~100%*2 chars

In total each of your chars will get roughly the same exp/hour as you would if soloing, assuming they're both attacking. However, that's about twice the exp you'd get on a single char - and if you then pick your classes/talents/totems so that your chars boost eachother you really start moving.

And last but not least you're far less likely to wipe, and you gain rested exp twice as fast.