I wouldn't hook the controller to the house batteries too, you would be connecting the batteries together, and they would both discharge, either from a phantom draw or bad batteries. Although it would be possible with the right electronics and relays to isolate them, I don't think it would be worth it for a 15 watt solar panel.
I doubt 15 watts will cover the self discharge from just the starting batteries, remember, you only get at the most 5 hours of full charging from it, plus the losses from the wiring and charge controller. A total of maybe 5 Amp hours of charge(probably closer to 1 or 2), versus 3 to 10% typical self discharge rate (per month) for a Lead Acid battery. Also, once lead acid batteries are left discharged below 50% state of charge they deteriorate very rapidly, hope you can figure out how to keep them up.