A unit frames addon, which shows target of party members is decent.
I'm using Pitbull 3.x.
But there are a lot of options out there in that regard.

Basically the main targets something.
You press your assist key, which has the slave ask the server what the main is targeting, the server then responds, but that is roughly double your latency as a delay.

If you switch from one target to another quickly, you'll basically have double your latency as a delay, before the clones/slaves are on the same mob.
If you set focus (pvp for example) on all of them for one target, you can dps another target... and the switch back to focus is virtually instant, since each already has a focus target set and knows immediately what their focus is.
But aside from targeting your focus (or casting at your focus), there's the delay between one toon selecting a mob and the others assisting the first toon.