Doing it with Y-connectors, as you propose, is possible. However, there will be volume control difficulties, particularly if the amps have different gain.
Doing it with a pre-out and daisy-chaining the amps is also possible, but both of the amps need to have a pre-out and power-in.
If both amps have a pre-out and power-in, here is what you do. Signal goes from source to amp 1, then from pre-out of amp 1 to line-in of amp 2, then from line-out of amp 2 back to the power-in of amp 1. If you only use the pre-out from amp 1 into amp 2 and don't bring the signal back into amp 1, you will not be biamping. The power section of amp 1 will be bypassed and unused when you use its pre-out. You will have pre-amp/power/amp set-up. It will identical to using a single integrated amp, only with the addition of another interconnect.
Passive bi-amping is too much trouble and money for too little improvement in my view. And it sounds like your amps probably don't both have all the ins and outs. Better to have your money in one good amp rather than spread the same funds over two lesser amps. Active biamping is a different story however.
