if two amps were putting out 'exactly' the same signal, then joining there outputs would help with the currant drive ability. such a concept is what naim amps are all about. They have excess outputs shareing the load. There just one amp, using one power supply though, and sharing the input signal. Your unlikely to find two separate amps with a close enough match in output to do the trick. You would be better looking towards custom pre-amps that allow the amps to bridge into monoblocks. I dont know if they exist commercially. Walk away lol
You probably have one set of posts because the designers knew better. Not all designs want bi-amping due to crossover concerns. They probably have a simple single capacitor design, set as low as they want it to be.