Many racks will have one shelf that runs a bit cooler than the others. Put your BRB in that one.
Ambient temps over 85*F can be a problem for a BRB. Belly heat at 88*F will get used but you won't find your BRB coiled on it, he'll stretch his belly across it for digesting while keeping his head and maybe his tail in a cooler area. You may also want to use a substrate like cypress mulch in your BRB's tub and paper for your ball python. I've found the density and dampness of the mulch helps to dissipate heat. You can also offer a large water bowl; if you find your BRB soaking a lot he may be too warm.