Hi Alphapi and
to the forum.
As most boats that I've come across seem to have an unlimited supply of cool water all around them, it might be better to use a liquid cooled heatsink plate to cool the controllers and the motor, as liquid cooling should be far more effective than air cooling, especially when the cooling air temperature is already at 50°C.
Regarding the connections for dual controllers
(where each controller is connected separately to one of the motor's two stators) you will need to make sure that they both share a common ground connection
(Battery -) if they are being fed by separate batteries.
If they are both connected in parallel to a single battery, you just need to connect the other control wires in parallel,
with the exception of the +5V supply to the throttle, which should only be connected to one of the controllers and the unused wire should be safely insulated.
I seem to recall that the main reason for this is to prevent the two internal +5V regulators from interfering with each others operation, as their individual regulated outputs are likely to be slightly different.
So, the e-lock wire, Reverse wire, Cruise wire, Throttle signal wire and Brake signal wire
(if they're used) on one controller would simply connect to the corresponding wires on the other controller.
You would also need to make sure that the controller parameters were set exactly the same for both controllers, bearing in mind that the Maximum Battery Current settings for each controller should be set to half the maximum current available from the single battery.
Alan