Hooking up two amps on one sub is a bad ideal. If the sub is a dual voice coil, then you can hook each amp to each voice coil of the sub. This is bad for the sub because the amps will not put out the same watts. Dual voice coils have a RMS values, most of the time each voice coil gets half of the rms. So most likely it would over power the voice coil.
it really depends on the amp i think, if they are good and regulated, then i don't see any problem (JL for example). but if they are cheap (sony for example) then who knows what they will do