I have room for 2 5-gallon fermenters (ale pails) in my fermentation chest, and would like to conduct two fermentations at once. Typically, I insulate my temp controller's probe against a fermenter, so it is regulating fermenter temp and not ambient.
If I want to regulate temp on two separate fermenters, I would think that the best way to do this with my chest freezer/temp control set up is to suspend the probe inside the fermenter so it regulates the ambient temperature and have it set 5-6 degrees below my desired fermentation temp (in this case 65 degrees, so I would have the controller set to 60 degrees).