Unless you have a very high end side-by-side, there is only one compressor in that unit. This means that only the freezer has cooling potential and the temperature of the fridge is adjusted by variable vents.
Here is my idea of how to make this concept work:
Basically, you need a dual stage controller like the LOVE TSS2-2100 described in my side-by-side thread in my sig. The 16A relay will control your compressor to maintain your desired serving temp in the freezer, and the second relay will control the fan. The tube connects the fan at the bottom of the freezer to the top of the fridge zone. This way, the cold air from the freezer will not "spill" into the fridge and you should be able to maintain separate temperatures pretty well.