I've modified a side-by-side to control the temp on both the freezer and fridge side. The most recent discussion on this type of topic is in this thread:
Here is a link where I got the idea to do a side by side fridge:
This project is based on the project in this link:
Here's one of my comments from the first thread:
I bought a Jenn-air side by side off Craigslist last month and have completed the build. It works great. My fridge only had one thermostat that controlled the compressor. So I got a 2 stage love controller and hooked one up to the replace the inline thermostat. This controls the temp on the freezer side, because it is what kicks on the compressor. The other stage controls a 120V ac fan that is set between the freezer and the fridge. So this fan controls the temp in the fridge side. It works great so far, but of course with this set up the freezer side has to be colder than the fridge side. I can post pictures and a schematic if everyone is interested.
I didn't have much trouble with the wiring, but there is a little trick to get the fan wired up correctly. For me the side by side is great because I can get 4 carboys in the fridge side. I wanted to be able to get 2 carboys in the freezer side, but it just isn't wide enough. I bought four 3 gallon better bottles that fit great in the freezer side, which allow me to lager two 5 gallon batches in the freezer side at the same time.