But you can buy 2 single-stage controllers for the price of one 2-stage. Then you can use them together or separately. http://www.pexsupply.com/Ranco-Temperature-Controllers-12744000
Where do you place the sensor? What differential do you use?
I ferment in glass, and put the sensor in a thermowell that passes through the stopper. So I'm measuring the beer temp directly. This also makes it possible to use a 1 degree differential.
So, having just 1 controller means just 1 probe.
This is not to say, though, that this is the only way to roll. Plenty of brewers just tape their sensors to the side of the fermentor and cover it with some sort of insulation, and get good results that way. If you take that approach then having two separate controllers would work just fine, I'm sure.