Save yourself a headache and order one of these:
http://www.reptilebasics.com/Ranco-E...d-p-16172.html
or http://www.reptilebasics.com/Johnson...d-p-16178.html

It's $80, but it's worth it... trust me. Think of it as an investment for your snakes. Plus, you can use just one for all your BP's. However if you use two pieces of flexwatt (one for hot and one for cool), you will need 2 separate thermostats.

Also, I'd probably put the tubs on separate shelves/surfaces instead of on top of one another, because the heat from the sandwiched flexwatt is being divided between the two tubs, making the bottom one too warm and the top too cool (most likely).