I have a 5.5 ft male brb in a 36*36*18(HxLxW) and trying to house two would be very cramped.

I know they are young but dividing the task in half would probably not be enough room even now. It is suggested to provide a snake with an enclosure that is 2/3 the snakes length in length and that is 1/3 the snakes length in width.

Also trying to provide the proper gradient in a 18*18*36 would be very difficult. Though they do climb some the majority of the height will be wasted. Your idea would work much better for a tree python or boa but is not best for a brb.

Maybe check out some taller tubs to give them some room to climb. Tubs are cheap and hold humidity well which is what a young brb needs. Then when they grow a bit get them nice pvc cages.