hi there
Food for thought here; it is true that we can run into problems using 2battery and 2 speed controllers - there is a lot of things that have to be taken into consideration though and the easiest way is to keep the explanations of it simple.
If we think of the speed controller being in 2 parts - i.e. front part and back part.
Front part - it receives a signal from the receiver and basically this signal in the front part is telling us what the 2nd part should be doing. So the front part of the controller controls interprets the signal.
Back part - this deals with the heavy amperage after its been told what to do by the front part.
These circuits in the speed controller are connected electrically - normally on the negative side and this negative line in turn goes back through to the receiver through the wires.
Therefore if running 2 speed controllers through the wiring, they will obviously be connected together and the receiver makes the loop.
When connecting the 2 batteries together supplying the 2 speed controllers the batteries negatives are also connected to the negatives on the speed controller side of the wiring.
Any difference in battery voltages could effect the speed controllers - through this built in circuit.
I hope this is understable
aye
John