Most older side by sides have only one evaporator, in the freezer. A duct and fan cools the fridge side from there. Creating more passages or using a bigger fan can keep the temps closer on both sides. Be careful, there are probably coils in the separation wall.
Newer models have 2 individual evaporators each with their own thermostats (using one and the same compressor). You would need to rewire those if you want them to work properly. For example, you can insert an STC-1000 into each circuit.