Heres a thread on how to add an auxilary plug to the back of your radio:
http://www.cvoharley.com/smf/index.php?topic=10044.0

To answer your question simply. Yes, the leads can supply the signal to 2 amps without an issue. The only problem would be where you would locate the second amp. I am currently running 4 speakers in my fairing and have 2 amps in there also and can tell you that it is a real tight fit.
Plans are to install the second amp in the tourpak.

 

Then you shouldn't have an issue. If you are running the aux outputs make sure you use a good heavy guage cable to run to the back amp because there is alot of length there. If you are running the speaker level outputs, once again use a heavy guage speaker wire to run back there also. I always run my audio a little on the overkill side with wiring just so I won't get any power or sound loss. I never use anything less than a 12g speaker wire and run nothing less than an 8g power wire for the amp.You could probably get away with less but for the extra couple of dollars it's worth my piece of mind.