I have checked all local laws. I am good with tandems to 70ft here in CO and most bordering states as well.
The reason for two brake controllers is to allow adjustment of the rear trailer in tow to have more braking power than the 5er to help control sway. I currently have a Prodigy brake controller which is good to 8 brakes, but that isn't really the point. I want to be able to adjust each trailer individually to help straighten the load if needed. And, to allow the rear trailer to have slightly better braking than the trailer in front of it.
It also eliminates the amp draw fall off through the wiring being 60+ feet long.
It also gives me a backup controller just in case I lose one while towing. Stopping 24,000lbs with just the tow rig would suck. Especially running over Loveland and Eisenhower passes here in Colorado.
I was just curious where to tie the controller into. I will be using the brake switch at the pedal for the 2nd controller.