Here's the key: I think if you have a voltmeter attached to the starting battery and begin to overwhelm the charging system then you'll see the system voltage dropping below 12.5 volts, at this point the starting battery will begin to discharge as the rectifier/stator cannot keep up.
I also think if your stereo + ski draw in total less than the wattage spec for your stator charging system, it should work fine with just one battery. The risk is discharging the single battery while the engine isn't running or it's running at slow speeds.
Watts = Amps x volts, so if your stereo consumes 240 watts constantly then the amperage draw would be around 240W/13.8v=17.4 Amps, a constant load like this is probably overloading your charging system and drawing the remaining makeup current from the battery. I bet the system voltage would drop to less than 12.5volts under these conditions.
So, just watch the voltage and turn down the volume as the system voltage begins to drop to 12.5 volts. You'll have more power as engine speed increases, I wouldn't try to exceed more wattage than the stator/RR are rated, plus a few amps for the ignition and gauges.
In the case of the 951 carbed motor, the charging system is rated at 180 watts, which at 12.5 volts is around 14.4 amps. Figuring probably 4 amps are necessary to power the ignition and gauges(estimated, not actually measured), then you'd have nearly 10 Amps remaining power, which is 10A x 12.4 volts = about 120 watts.
I doubt a stereo will draw this much power unless you play it annoyingly loud, perhaps I should tell you you cannot add a stereo to you ski?
