If you wire that 2nd battery to the original battery in parallel, they become one larger battery. So at the end of the day, your amp would still be running off your one cranking battery.
I am a HUGE fan of having 2 batteries in a boat, but not wired together, but rather as separate banks isolated by a manual switch, etc.
The only thing "wrong", if one wants to use that term, with having a stereo draw off the one cranking battery, is it limits your play time while at anchor. Other then that, there is nothing electrically wrong with it. A dual battery setup simply allows us to keep a battery in reserve, and play the tunes while the engine is off.
Sorry for the added info, I realize its not actually answering your original question.