Garry, the key phrase here is series vs. parallel. No, you don't want to be series connecting your trolling motor battery to your engine starting battery. That said, a properly connected battery in parallel will work nicely. Just understand that the starting battery will recharge much more quickly than your deep cycle unit. Also, you don't tell us what the technology for the deep cycle battery is. If its a gel cell, you probably don't want to connect it to the outboard at all. If its a flooded cell or an AGM, things should work out here.