You could try connecting the headphone output of one to the aux input of the other. That won't get you a stereo pair just both playing the same source.
Any modifications will likely not work well if at all. Bose doesn't in any way over engineer their products so adding more speakers could blow the amp and would certainly not have enough power to drive them well.
If you want better sound it's time to get rid of both Bose and get yourself a real audio system. It won't take a fortune to get better quality of sound and more flexibility too.