One of my first teams was a protpala+3afflitionlocks+1shadowpriest. They were the 'plague brigade'. When they reached 70 I splitted them and put them in different teams. They respecced destro and everything changed. Decent DPS, they've never had when Afflition specced.

By my experience, I found Destro Locks much better to multibox then afflition on PVE content. Most of the time you need burst and quick direct damage, dots don't help you too much when you run a dungeon with your tank on aoe mode. Right?

About combos and box-friendly things, I've just post something on other thread that may be interesting to help you to find your ways.