One of the macros from the stickies works well for this. I use it extensively in combination with a normal /assist focus macro for swapping targets.

Useful multi-boxing macros

Relevant macro:

There is one other variation I've thrown into the mix:

/cast [exists,harm,nodead] fireball
/stopmacro [exists,harm,nodead]
/cast combustion
/stopcasting
/assist party1
/cast [exists,harm,nodead,combat] fireball; [exists,harm,nodead,nocombat] pyroblast
This will allow you to acquire your party1's target and proceed to cast pyroblast first followed by fireball until the target is dead. Once the target is dead, it will assist party1 to get a new target and again cast pyroblast followed by fireball until the target is dead.

In my case, I edited for use with my shamans and to use my focus as assist instead of party1:

/cast [exists,harm,nodead] Lightning Bolt <----Second and consecutive spell cast on existing target or first spell cast on any manually acquired target
/stopmacro [exists,harm,nodead] <----Stops macro from executing the remainder of the commands if you have a target, it is an enemy, and it isn't dead
/assist focus <----Acquire the focus' target
/cast [exists,harm,nodead,combat] Lightning Bolt <----First spell cast on new target

This allows me to acquire a target with my paladin; get the shamans casting on that target; swap targets with my paladin to build agro or stun it; all while the shamans keep on dps'ing the first target. When the first target dies, the shamans will automatically swap to the new target and keep on casting. The paladin can switch to a new target once the shamans have started casting on his current target.

If I need to switch the shamans over to a new target before their current target is dead, I use the standard /assist focus macro. I also use a /cast [target=focustarget] Frost Shock macro to kill runners and a /cast [target=focustarget] Earth Shock to interrupt casters without having to swap the shamans off of their current target .