If you were to do this, connect the positives and negatives together so they are hooked up in parallel, 12 Volts (versus in series, 24 Volts). However, do not hook together a deep cycle to a cranking battery, very dangerous. You shouldn't even hook up a new and an old battery together.
If I needed more juice I would buy a deep cycle battery and run my live well and electronics off of it. Leave your cranking battery for just your motor and then it should provide enough power.