Don't do it! A "Y" connection will short the two amplifiers outputs together. While preamps are unbothered by this which is a way to get a mono signal (they are short proof) power amps are likely to burn out.
4 responses Add your response
A true subwoofer for your application will have a dual voice coil (right and left). Go to partsexpress.com or mcmelectronics.com and buy one for $10 and you will do much better.
The other option is to get a simple sub crossover which will filter the bass out of the 2 mains and send it to a single subwoofer. I've got an extra crossover if you want one? It will run you $10.
I'm out of $10 options...