Alpha, what you're asking about is called "passive bi-amping", where the speakers' crossovers are still in the audio path and still 'crossing over'. A simple Y-cord will feed the two amps per channel. They need not be of the same rating, although it wouldn't hurt; if there's a difference, more power to the lows.
Keep in mind that you won't get an appreciable volume difference, as the two amps will still be putting out the same voltage across their terminals, and thus into the speaker terminals. Both amps still output the full frequency range, but the power falls off outside each speaker section's pass band.
The main difference will be the addiional power supply capacity. Now, if the amps were operating near their capacity when driven individually, bi-amping would improve things a bit, but contrary to popular belief, you don't get twice the power. That only comes from increasing voltage, and thus current.
Bridging is required to really increase power from a given amp. Main requirements are 1) a phase splitter (I have a simple circuit), 2) the two amps must have (or be able to have) their speaker negatives tied toogether, and 3) they must be able to handle impedances half that of the speakers.
The next step upward would be to use an active crossover, which should be accompanied by bypassing (removing) the speakers' internal crossovers from the signal path. This is an "electronic version of a Y-cord", with brains. The pass bands are divided before the amps receive the signals.
In my old van, I had a 4-way active crossover driving a 35w/ch amp on the tweeters, a 40w/ch amp on the mids, a 55w/ch amp on the woofers, and 130w mono on a 12" sub in the rear, plus a second battery right next to the amps. Nothing but wire between amps and speakers; it sounded great!