get a Y cable to take the stereo output into a single channel of the amp. Run mono.
Connecting both amps to one speaker is not significantly from simply wiring the two amps into each other with no speaker at all. Bad smells and dead amps ensue.
The extra power, if you used it, would add 3 dB (about one notch louder) so it's not that big a deal.
The speaker's power handling is, like most, probably a bit optimistic. Moreover, those are usually thermal (will it melt) numbers and the speaker is distorting and compressing (stops actually getting louder) somewhere below half of its thermal capability.
The speaker doesn't need the power. You understand more power means louder, it appears. That HAS TO MEAN that if the speaker is not ALWAYS AS LOUD AS IT CAN GET, sometimes it's receiving less than full power. In fact, even outdoors, a couple of watts sounds real loud if you're close to the speaker. If you've never destroyed a speaker by turning down the volume, you have about all the proof you need that the pspeaker doesn't need full power all the time.
If the one speaker isn't loud enough, you'd get a lot mlre kick (actual SPL increase) by adding another speaker than by adding power . . .