You can underpower a sub all day long and not cause it any damage PROVIDED you allow the voice coils to cool after sustained use and do not clip the outputs of the amp which then heat the voicecoils again. As far as the two amp route goes, not the best idea. You now have two amps that will draw more current than one large one, they generally cost more than one large amp, you double up on the real estate taken up, your wiring costs more and finally it becomes a real beyatch to PROPERLY set up.

You can overpower a sub as well but this is far more dangerous to do. Heat is the #1 killer of subs. Too much power on tap, well sure the sub can handle it in bursts, but breach the thermal barrier and your speaker is a new doorstop.

You have lots of options on Class D 1 ohm mono amps. It depends what kind of amp you are after and the budget you have for the amp.