David, I do not think that a thermostat will be able to do a rack and a cage on the same stat, at least not to any degree of accuracy. In order for them to be accurate, it has to be the same type of heating element... i.e. two of the same type of heat pads, or two racks with the same amount of heat tape. In order to do both the king cage and the racks, you will likely need two thermostats.
Are you at least using rheostats? You can use $8-10 plug-in style lamp dimmers from the home depot to keep the heat pads dialed back, which is a necessity... an unregulated ZooMed heat pad can quickly soar to 115 or higher.