The only problem I can see is that you'll be limiting the trolling motor range. Otherwise 12v is 12v whether it comes from the cranking or deep cycle battery, but the cranking battery will recharge when you run the motor.
Can you do it? Yes but as said you would be getting less range from a charge. I run everything but the trolling motor on the cranking battery and a separate battery for tolling motor only.
On the smaller size engines you do not need a true cranking battery. The reason some do not want to hook radios to cranking batteries is if they are true cranking batteries, they won't hold a good charge like a deep cycle.
You should be able to use two deep cycle batteries which is what I do. Guess I wasn't really clear.
thread is older, but have you thought about using a switch? Then you can use the second battery for trolling or music, switch to the first to crank up the motor, switch it to both to charge them both from the motor.
