It would be an interesting experiment to compare running one vs two motors driving the boat at a given speed. It's likely a wash in terms of loss in the controller and given near identical prop characteristics between the two, you're talking about doubling the prop loading in the 1-motor scenario compared to 2. At low loads (and speeds), that might not amount to much of an efficiency reduction to drive with just 1 prop, and might even result in the prop operating closer to its higher efficiency zone (Gerr's prop manual might inform here). But as the boat drag goes up with the square of the speed, at the higher speeds, having the dual props and dual motors sharing the load likely wins out big time assuming we're talking about speeds where the loading nears the high end of the motor's rating. And motor torque via motor current delivers that force.
Motor heat loss (2mtrs) ~ 2 * k2 * Rmtr * (Fdrag/2)^2
= k2 * (Rmtr * Fdrag ^ 2) / 2
Motor heat loss (1mtr) ~ k1* Rmtr * Fdrag ^ 2
Assuming k1 ~ k2 (conversion losses in controller, prop loading, etc. are near same), we should expect the single-motor configuration to have about 2-times the heat loss as for the 2-motor case.
So even if prop efficiency for the 1- or 2-motor scenario were identical (unlikely) at the high speeds, we should expect twice the heat loss.
Now, how much of the total power to the controller(s) is lost to motor heat due to motor current? It's a lot, but the efficiency of the propeller, combined with the drivetrain is the shortest stick---this could be as high as 70% at speed or as low as 50-55%. This, compared to a difference between 90% vs 80% efficiency for a lightly, vs highly loaded motor. Controller efficiency is likely in the 95% zone. Cable and battery losses also factor in.
Add to this, dragging an undriven prop thru the water---best to drive that prop with just the right amount of power that its current draw just starts to increase. At that point, it is presenting minimal drag.
Bottom line: Prop efficiency is paramount. Loading the prop to the point that its operating point moves out of the higher efficiency zones quickly defeats other gains you might attain elsewhere (e.g. by driving with just 1 motor). Add to this, I^2R losses in the motor and battery/cabling quickly conspire against you. But it all depends on the loading. At low speeds and loads, the single prop drive likely wins out. As speed and load increases, single prop loses.
Still, let the data speak :-) .