You only need two heating sources when your room temps are too low to maintain a proper cool side...like here. Luckily for me, though, the cool side thermostat doesn't come on much in summer.

As for single tubs, the zoomed pad is just fine to heat them. Just make sure you have a t-stat to control it! Rheostats are a waste of money, and if you're going to spend the $20-30 spend it on a ZooMed ReptiTemp 500R stat instead...unless of course you have the money to get a GOOD stat right off (which you should try and do).
Just use foil tape to tape the pad(with the paper still on the adhesive) to the bottom of one side of the tub like this:



The same would be done with Flexwatt, then you just hook it to the t-stat and let it run a day or two with the substrate to make sure temps are stable.