I cannot claim expertise, merely familiarity with the construction of the 6 fridges in my house, as well as the one that I killed by mistreatment (incorrect PID temperature controller programming). All the combination refrigerator/freezers I own have but one compressor, and more importantly, one evaporator. The evaporator is the "heat absorber". The fridge settings allow direction of more of that cooling to the freezer compartment or the refrigerator compartment, putting a greater portion of the work towards one compartment or the other. However, the two compartments can't really be independently controlled. That said, you might find that if you really bias the controls in favor of removing heat predominantly from the freezer, then you can get acceptable lagering temps in the refrigerator compartment. It's not as neat as a separate fridge with precise temperature control, but it may be acceptable "salvage" of the fridge for brewing purposes.