More Power???

Assuming we are dealing with identical amps, would it not be better to bridge each into a mono amp? In such a case the power would be more than doubled. Someone more technically astute than me could explain why and how much power would then, at least theoretically, be available. I am not certain bridging is always feasible though. Sounds like what you are attempting is more like bi-amping, which can be done with a number of speakers; again this requires two identical amps and speakers that can be bi-amped (the crossover actually being in two sections that can be disconnected). Again someone with a better technical background than mine would be called for.

Bridging a stereo amp into a mono is not without compromise: damping factor typically degrades by half, which decreases bass articulation & control, perhaps even masking HF subtleties & nuances to boot. But other than bridging, there is no other method of achieving the aforementioned at least that I'm aware of.

Yes, it is possible to wire multiple identical amps up together in parallel to achieve greater power output while driving the same load. This is done quite commonly in the RF field and requires impedance matching splitters and combiners.

Yes, it is possible to use a smaller amp to drive ( run in series ) a larger amp. This too is done in the RF field quite commonly. Special attention must be paid to impedance matching and the input sensitivities of all the equipment being used. Otherwise, severe overdrive ( MEGA distortion ) or "self termination" within the system will occur.

NO, i don't recommend either of these types of installation for audio purposes. Blueswan basically stated the reasons why. There is NO reason to do such when there is an ample amount of reasonably priced high power / high performance amps available on both the new and used market. Sean
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I don't know what amps or speakers you are using so can't assess the desirability of increased power in terms of sound quality or volume. In my experience vertical biamping provides a huge improvement in sound quality without the compromises inherent in mono block operation. Requires biwired speakers. --Scott