Elec Trailer Brake Control - Internal / External

I want to have both trailer-mounted and vehicle-mounted options for electric trailer brake control.
My horse float came with trailer-mounted electric brake controller. I already have a vehicle-mounted controller which I would use in preference but I want to be able to lend the float to someone who doesn't have a vehicle-mounted controller.
I tried connecting the 2 controllers in series but they won't work that way - don't know why not. I can only think that the two control circuits must be incompatible.

The only other option I can think of is to bypass the trailer-mounted controller when the vehicle-mounted controller is in use. I'd rather not use a switch for this because that could be a bit risky if it is accidently bumped. I was thinking of doing it by wiring up 2 different plugs. One, 6-Pin Small, would be for trailer-mounted control, the other, 7-Pin Large, would be for vehicle-mounted control. But this could get pretty messy and will also require a special 6-Pin Small cross-over connector to connect vehicle-Stop to both trailer-Stop and trailer-Brakes when the trailer-mounted system is used.
There may also be a relay-based solution but that's a bit too technical for what needs to be simple.
Anyone else solved this problem or have some ideas?