The heating pads can be used, but there is a risk of accidental burns to the frog. They are burrowers by nature, and as such they will attempt to burrow down to cool off. There in lies the problem. Most people don't have problems with them, but there is a risk. As far as additional heating, I promise you that you don't need any. If you've got the temp at 80 degrees, or close to it, you're fine. Don't worry about the cooler substrate they'll be fine! And by the way, I hope that they're not housed together... Please post any other questions you might have.

Evan