The HUGE thing I've noticed about dualboxing is that I have nearly zero downtime.

With two mages the paladin was all but useless until I started doing AE Grinding. With the 4 warlock + priest group I toyed with I was doing level 9's at level 5 without breaking for mana or health. Just 5xnuke, repeat, move on.

Oh, and at level 5 I hadn't trained yet... so I was still slinging level 1 spells. Once I trained it was even more broken.