Tony,
I can't tell you what the problem is without more information. I need to see the electronic schematic for what components you are combining, and, in the case that you have a wiring error, I would need to probe your wiring with a VOM. No biggie; just details.
I don't know if you are using a bistable throttle, like you see on commercial scooters, or a proportional throttle controller like you see on a Zero. My guess is the latter, which would mean that you will need to diode isolate the two speed controller inputs from each other, or one will find an unintentional electrical path through the other.
Another likely problem would be that you think the 2 speed controllers are independent, but they are sharing the same chassis ground (metal frame of the bike) which can lead to unintended electrical paths.
But the most likely problem of all is usually that there is just a wiring error somewhere, or a short. I can show you how to find these things quickly with a voltmeter. But the most important thing is to know what we are looking at.
If you are feeding the same proportional throttle signal to 2 speed controllers -- even if they are precisely the same model of speed controller -- one will "current hog" and steal the lion's share of the electrical current from the throttle. Because of the non-linear nature of dominant current paths, current-hogging is an effect which takes place and causes problems even if you have perfectly matched components -- for example transistors or diodes. One will randomly draw slightly more current, and as it becomes dominant. it will become an even more efficient current path of least resistance, and hog even more current. Hence: "current hogging."
How is your project going?
