Looking at the photos of the PCBs I doubt the unit has any active current limitin. That said the 0.23 ohms you have measured is very much worst case. You have not included the resistance of the wiring, fuse (you are going to put a fuse in circuit?) or the internal resistance of the battery. Added to this the current will fall as soon as the motor starts to turn. The unit will almost certainly use MOSFETs. These genally have a puls rating at least 5 times their DC rating So may well be OK. That said itwould be safer to use one unit per motor and they are cheap enough.
You could use a single potentiometer for two controllers. Connect all three wires from one controller to the pot (short out the switch wires or use a separate switch, possibly double pole to be safe) and just the black and yellow wires (CCW and wiper connections) of the second controller. You will have to sort out the forward/off/reverse switch as well. It should be possible to use a single pole switch (the original) but I can’t tell without the unit os circuit. Worse case is use a double pole switch.

Robert.

I am a little confused by the extra switch – does it have two ranges?

Forward reverse support ?? eh?

But looks a useful unit ie cheap.

With all this rain, Ahead and astern may be more appropriate ? Noel.