You only need a single-stage controller if you are only heating. Same if you are only cooling.
One stage turns something on, when the temp crosses a threshold. Two stage turns Thing 1 on when temp is above the threshold, or Thing 2 when the temp goes below the threshold.
I.e., here in florida now it gets up to 80 during the day and below 60 at night. If I want my fermentation chamber to stay at 65, I need to have both cooling and heating. So, a 2-stage is perfect: Both my freezer power and a heater are connected to it. It's set to 65 (that's the threshold).
One thing to point out. A poorly designed 2-stage controller might switch a lot at that threshold. You can imagine the temp getting to 65.001 F, the freezer turns on, then 1 minute later it's at 64.999 and the freezer turns off and the heater turns on, and this cycle repeats for ever. My freezer would die pretty fast. That is where hysteresis comes into play. Hysteresis is the range (i.e., 3 degrees, from 62-65) where nothing happens. Essentially there are 2 thresholds, 62 and 65. Heater turns on when below 62, freezer turns on when above 65.
I should note that I only have a single stage device now. I need a 2-stage.
