When I first tried multiboxing it was with 2 shaman and while they worked well together I found that they weren't nearly as OP as I had expected given that there were two of them. I think shaman have a serious synergy when you start collecting more and more in your group due to the different totems piling up.

I then tried a pally/mage combo and they're amazing, definitely my favourite especially when it comes to AoE killing. The pally runs around gathering aggro and starts consecrating and puts on his ret aura then just stands there healing himself while the mage drops AoE death on the mobs. Works amazingly well.

Then I tried a Druid/rogue combo. . . tough combo.

Now I'm running a lock/priest combo and they work really well together. I use my voidwalker to run around an gather aggro like the pally while the lock/priest drop DOTs on each mob in sucession and watch them all fall over dead.