There are brake controllers that are rated for 2, 4, or 6 brakes. There may even be some for 8 brakes.
What is your current brake controller rated for? In order to work properly without burning out, it must be able to carry the electrical load for ALL the brakes installed on your double tow. If you will have 8 brakes, but your controller is only rated for 6, you will need a bigger controller or use two different ones.
I would use 10 or 12 gauge wire for the run to the brakes on the second trailer. The simplest and easiest would be to splice into the brake wire at the rear axle of the fiver. That may not be the BEST, though.
Good luck.