I have mailed Kelly and apparently one of their pre-assembled kits uses two controllers with a single pedal.
As suspected, this is achieved with just a shared GND and using a single one of the 5V outputs of the controllers. The pedal output can then be shared between the two.
My worries were more related to the fact that the controllers will never exactly "see" the same throttle and if this could create issues.
In our case, the motors will be coupled on the same axle, so no problem I suppose. If there was one motor for each wheel, torque could be different on each side and result in a a tendancy to steer.
That's all I know right now
We'll see how it goes.