Which would be more important, deeper substrate or a pair of caves (one on the hot side and one on the warm)?
You are going to want both ideally. The deep substrate is good for burrowing (which they will do) but not deep enough to cut down on the heat. I would say at least 1 hide and deeper substrate is probably fine but two hides are good for most terrestrial snakes. According to the care sheet sand boas also destroy their environment so dont get a ton of stuff in there and also dont get heavy hides for when they burrow so it doesn't injure themselves