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.

What would the advantage be of adding another crossover (the capacitor) ahead of the HF input of the speaker?  

I think the OP would need some decent measurement gear to get that volume control set correctly.

I still am of the view that bi-wiring with the best amp rather than bi-amping with different amps is better approach, but I wish the OP success regardless.