well there are a few variables that go into whether or not a single thermostat can run more than one rack.
1. heat tape: all of the heat tape connected to a thermostat must be the same wattage and size.
2. Rack configuration: The racks must be identical
3. You can't have more flexwatt than the thermostat can handle... when you see flexwatt listed it usually has a size and a wattage. (such as 3 inch 6watt) This means that for each foot of 3 inch flexwatt it requires 6 watts of power so multiply the wattage by the total length of flexwatt then subtract it from the wattage of the thermostat.
There are other options as well such as the Herpstat 2 and 4. the herpstat 2 has 2 independent thermostats in one box, and the herpstat 4 has 4 independent thermostats in one box. this is a much more cost effective thermostat style and will give you the most flexibility and ability to expand in the future.
http://spyderrobotics.com/
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