this is rediculess, you'll have to relay isolate each speaker wire so that thier is no cross feeding of the amplifiers.

dont do it, if it was dvc id say go for it, but your two amps, (if not equipt for slave mode) are going to send differant signal at differant times.
if 1 ground is longer than the other the amps are going to run differant current.

I would take totalimortals' advise and buy an amp that is designed to supply you with your power that you need.

otherwise plan on throwing both your 300w amp out, cause the channels are not going to handle the back feed.