Nathan has used eggcrate to separate his boys.
You can see it in his post here:
http://www.redearslider.com/forum/viewtopic.php?t=14842
Keep in mind that a 40 gal won't last too long, especially with 2 RES. Nathan's is a 240 gal.
With the tank divided, you will still have to provide 2 basking areas. For a 40 gal, if that's all you can fit right now, I would suggest building an above tank of some sort, so you can put as much water as possible in there.
At some point in the future, you will need two complete setups, or one monster tank. If you can't see yourself ever doing that, then maybe it's best to rehome one of them.