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I too am considering this question. I've been told that preamps are generally wired inside by splitting the outputs, not by laying out separate circuitry for each output. If that is the case, then a Y-adaptor should be the way to go. If you talk to interconnect companies and salesman they all tell you to buy another pair of interconnects. In your case, with only one set of balanced outputs, I'd definintely use Y-adaptors.
Using two amps running full range into passive crossovers is NOT bi-amping. Sorry. Bi-amping requires bypassing the original passive crossovers and using an active crossover between the preamp and power amps matched to the design crossover frequencies and slopes as the original passive crossovers. True bia-amping provides multiple benefits - up to 6 dB improved dynamic range, amp-driver optimization, better driver control from removal of passive crossovers components. What you propose provides none of those benefits, but all the expense and additional complexity.