To your original question, I don't see any danger in doubling up. As long as both heat sources are regulated by a thermostat, I don't see a problem. Did your friend explain why doubling up is dangerous?
I still think, even with the back heat, that your temps are too low. Even if your snakes don't get sick, I'm concerned that they will spend all their time holed up in their hides. On the other hand, you are keeping a close eye on them, and I'm sure that, if problems arise, you'll address it promptly. Welcome back to the hobby!