This may not apply as much since I'm only 2-boxing, but I have a warlock and a mage. Right now they're only lvl 21 each. I have key 2 bound to a cast sequence on my warlock (the "leader") that goes Immolate,Curse of Agony,Corruption,Drain life, Drain Life, Drain Life. It's also set for [nochanneling] so I can spam it without restarting the drain life. On my mage, key 2 is a simple /assist leadername /cast fireball. So as I sit there spamming my warlock macro, my mage happily peppers my target with fireballs. Yes it's simplistic but so far it works for me. I'll be redoing the mage macro to add in Pyroblast if it's up.

Edit: I should note that I'm new to Multiboxing so I'm keeping things simple as I learn the ropes.