Sounds like a great project.
Do the trolling motors go as fast forward as they do backward? My only thought is on skid steering rovers the motors normally operate at the same speed either forwards or backwards mean you can turn on the spot. If the trolling motor is more powerful forward then in reverse when you try to steer it might not turn very sharply. Its not something I've tried. Just thought I'd mention it. I guess the mounting of 2 motors is simpler then attaching a servo to each and using them for steering in the traditional manner.