Other than hitting tab REALLY fast... you would have to write a mod most likely. One that used a chat library to send names of targets to each other and if it was different, keep retargeting.

Which may or may not even be possible.

Another option is to have each character tab different amounts, sort of like how you can move in different directions with the "same" key press.

So have A tab twice, B tab three times.

Just be careful in anything but PvP.... you will probably pull the entire instance that way.

In PvP though.... you could PoM / Sheep 4 targets..... would make arenas quite interesting.