Your controller probably already has a shunt in it that you can just tap into, which it uses to limit battery current. Keep in mind that if you tap into it with anything other than a very high impedance, it will probably affect how the controller itself reads the shunt, and may affect it's own current limiting.
Typically it just looks like a thick silver wire (sometimes multiples in parallel), usually at the same end of the board the battery wires come in on. If you post some pics of your controller's PCB parts side, we can point out the shunt to you, if you don't see one.
You usually have to tie into the throttle because unless you have direct access to the proper parts of the controller's circuitry (often not possible because it's inside the MPU) and know exactly how it works, the throttle input is the *only* control input you have that can be used to tell the controller to use less current than it is using now.
The only other option that most controllers have is to disconnect the shunt's inputs to the controller's MCU, and stick a circuit in between the two things that lets you alter the amount of current the MCU *thinks* is being processed, so that the MCU then does the throttle-down/etc. behavior by itself in it's own way.
Based on your post here:
http://endless-sphere.com/forums/viewtopic.php?p=480322#p480322
quoted below, I think the principle behind what you are wanting to do will work (I don't know if the exact circuit will work or not), but I think you are probably over-thinking it.
If I understand what you want to do correctly, all you really need to do is put an adjustment pot into each signal path from your single throttle to each controller, so that you can set one to respond more to the throttle than the other, if necessary.
i got some 10k audio taper pots so its more of a ramp up control instead of a linear control. either way, i couldnt find small Ohm trim pots at radio shack and i realize i can put larger resistance pots on there and just dial them back but if i want a potential 0-100% limit could i just put the one 10k pot in there? also, i saw something about opening the controller and just adding a pot in parallel with the resistor thats already in there so adjust for more resistance. if i just did that i wouldnt need anything else in there right? i will be using 2 controllers spliced to one throttle with diode limiters and im thinking i can have a box w/ 2 knobs and 2 leds (for standby) one for front motor, one for rear motor. so in theory i could set say the front motor so be barely receiving any power and the rear motor to receiving full power if i like (even though i'd probably just have them relatively the same) and if the pot is turned off (theres a built in switch) it would just default to the stock resistance because the other resistor will still be in-tact. would this work?
Regarding this part:
if i want to build a module that will limit the max ammount of current that can go through the circuit then couldn't i just put a pot in there in line with the negative battery terminal?
the answer is yes, you could. But it will be a giant wirewound pot that weighs as much as your whole bike and batteries and motor.
(not quite that bad, but probably 15-20lbs)
