Bi-Amping Which amp for highs and which for lows?

My rule of thumb: Whichever way sounds best!

Pick the better amp of the two and biwire rather than biamp.  

If you decide to go for biamping have you confirmed that both amps have the same gain?

Unlikely they have the identical gain, so padding one down would be necessary.  You could use a high quality capacitor in the high input to roll off the bass, and use a volume control of some sort in the bass input to match the output.