the closest thing to strapping non-strappable amps I would even consider would be something like bridging them each to run one voice coil of a dvc 4. Gain and x/o settings would need to be as close to the same as possible.
Even then I would never plan a set up like that - more of an experimental type of thing.
U need a Bridging Module! And only certain amps will be able to work. (strappable) Its a wayy u can check. But I forgot. 1 amp amplifiys half signal and the other, the other half. Doing what Optidriven said is really putting each amp on its OWN voicecoil. Same SUB 2 amps! ^^ thats straight competition!
Other than that, u need a bridging module 2 hook 2 amps together... hope that helps.
This is how you check to see if your amp is strappable:
Being that some monoblocks are 2 channels already internally briged, these can't be externally strapped...
The way to tell, is with a DMM; measure from the negative speaker terminal on the amp, to the battery negative terminal on the amp for continutity... if its connected, then you can strap the amp, if its not connected, then it means its two channels internally already bridged, and the speaker output is floating, and can't be strapped!
Strapping happens when you reverse the phase of one of the inputs, then one amp basically drives the positive side of the wave, and the other amp drives the negative. When 2 amps are strapped, you use the positive terminals of both amps, and leave the negatives unused..