The divider (and the rest of the cage) is manufactured using a computerized milling machine. this means that they can make extremely precise parts. The divider simply pops in. no grove, or anything else. this is nice because it means you can split the cage in any ratio that you want (50/50, 60/40, 70/30, etc...) AP at least does offer the option to cut a slot for the divider, and include screws to make it permeant. (I might be getting a cage like this next time I order one)
RHP's when using the divider becomes an interesting situation. the best option is to just get 2 smaller RHP's. But you could cut a slot in the divider and mount one larger RHP in the center as well. Make sure that the slot is cut deep enough that there is some air space between the RHP and the top of the divider, as RHP's can achieve relatively high surface temperatures. (make the slot as deep as you can, without allowing the snakes to get through it. I would say 2-3 inches would be enough)