What I do is use a 2nd heating pad, but hook it up to a rheostat before going into the first thermostat. This way it's indirectly controlled by the thermostat. I've had nothing but good luck with that.

Another option if you can't get that rheostat/dimmer is to put the cool side heating tape an inch or so below the tub and then the warm one directly taped to the tub. Sticky furniture pads work great to raise the entire tub up off the surface and then you can play around with heat tape positions to get variance that way.