What does the freezer do....

on a fridge with a temp controller? Does it stay the same temp? Does it maintain the same temp differential as it normally would? I was thinking it would be possible to lager ferment 52f-55f in the fridge side, and lager 32f-34f in the freezer side. Of course you would need a side by side fridge to accomplish this. Am I nuts, or is this feasible.

It might be possible, but controlling temperatures for both compartments is difficult at best and impossible for most units. You have to chose one compartment for the temperature sensor and fiddle the internal controls to adjust the differential. Some fridges do not have any way of changing the differential, some do.

The evaporator in a side by side is in the freezer. The freezer temp in a side by side is what determines when the compressor runs. The temp in the fridge side is controlled through the use of a damper that allows cold air from the freezer to circulate through it when the evap fan is running in the freezer.

You could control it with dual temp controllers by doing this:

Put one temp controller's probe in the freezer side. Plug the fridge/freezer power into that controller.

Find a way to close/block off the damper that johnsma22 mentions (usually vents in the back of the fridge near the top). Carefully cut a computer muffin fan sized hole in the wall that separates the fridge from the freezer. Install a fan and use the second controller to make it run. You may need a second fan pointed back into the freezer side for it to work properly.

Of course, it'd be smart to just test run the thing on a single temp controller with the probe in the freezer side before making any drastic mods. Perhaps you don't need to do any cutting at all!