I believe that just twisting together the cables will cause one output pre-amp to drive the other. ie. the front left pre-amp pushes a signal back up to the rear left pre-amp.
A balancing amp would do this for you but for what?
If you're using an external device for crossover functions, just run the front outputs of the empeg straight to your EQ/Crossover. this would give you the three channels (F/R/S). You could still use the empeg rears as another sub/rear output with another amp if you want, but getting the balance right would be a bit more involved.