Speaker wiring question

I have a pair of outdoor speakers for my deck that sound OK, but they probably only go down to 80 or 100hz. I would like to try to supplement the low end with a single bass driver from an old speaker as a makeshift sub wannabe. I have connected it with speaker wire from the B speaker outs (an old Yamaha receiver) left channel, and though not perfect, it does improve the bass output. My question is, can I connect it with both left and right speaker wires akin to using a Y connecter for a real sub, or am I likely to be sorry I tried? I am assuming that it will not affect the A pair of speakers. Thanks.

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.
Not sure what connections you are talking about?
Thanks for the caution. I'll settle for the way I have it.

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...