Overall efficiency depends on several factors, including the motor controller(s), the amount of drag in the water, and the propeller. It's really helpful if you can measure the power used by your motor(s) in different circumstances, and when trying out modifications.
Trolling motor controllers vary widely in efficiency. The new electronic ones are a huge improvement over the old-school resistors and switch type.
When you start using trolling motors for cruising instead of just positioning, then the drag of the motor and shaft becomes a significant factor in the overall efficiency. I use a 600W Haswing Protruar with my little trimaran, and I was able to reduce the energy needed to go 5mph by nearly 20% by adding an airplane fairing (for landing struts on kit planes) around the motor shaft, a nose cone, and an 11x6 APC propeller. You might want to look at sturdier after market trolling motor propellers for your heavier boat.
Another advantage of two electric motors is that you can have one that works best for fishing, and the other that is adapted for cruising. The fishing motor can also be a backup or augment the cruising motor. It sounds like you'd like them to share the same battery, so I think other things being equal, using one motor is more efficient than using two. But the benefits of having two motors available have already been stated.
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