I've been dual boxing a pair of mages up to lvl 13 so far, I've certainly noticed that the main toon does more damage according to damagemeters (synched) but so far they have dinged identically... very cool to watch ^_^.

Just make sure you've invited the 2nd toon into a group before you start nuking mobs... I always forget when starting out each session and have to solo the alt toon so he can catch up.