I used that method for a few years, when I had a single ball python in a tub. Our house was kept relatively cool, and I wasn't going to heat an entire room to achieve the right ambient temps in a single tub. A plastic tub does not work well with heat lamps either. So, the method that seemed most reasonable to me was to have two different strips of heat tape, each hooked up to its own thermostat.

I used 11" for the "cool" side and kept it at 80 degrees...and 4" for the "warm" side, set at 90. Never had any issues with it.