Bi-amping isn’t something you do half-baked. Y cables will attenuate the signal somewhat, but the real point has been made above. If you are going to biamp, you should be using an active crossover between your preamp and the power amps. That is the correct "Y cable."
I have tried the method above and it may sound cool at first but you will soon realize the imbalance in power between the monoblock pairs unless they are identical. Good crossovers have gain adjustment to get it right.
I would look for a good analog crossover. Marchand makes some decent ones without being too budget-crippling and Phil will answer your questions directly.
I should also say that bi-amping really only makes sense if you can bypass the crossover in the speakers.