Other than avoiding two different melee types in a 3-toon combo, I wouldn't have thought there'd be too much issues with class combos, as I can see you've got at least one 70 in each account for some boosting.

Personally, I reckon if you're going so far as 3-box it, one or two steps more (and a second monitor) puts you into 5-boxing it and running you're own heroics.