almost all fridges work on a 2 stage system but only actively cool on 1. in a typical top/bottom, the freezer has the cooling coils. the fridge temp is actually just controlled by a damper that lets cold air sink into the fridge portion.
on a side by side, it's a fan or damper system to make this same thing happen. the freezer side has active cooling, the fridge part just gets cooled from the freezer side.
so if you only have 1 temp controller and don't want to punch holes in the whole thing, you should ferment in the freezer side, as it should be more energy efficient.