To explain your situation for future reference, you have a head unit with a single set of RCA outputs that are directly controlled via the sub output control. Since you had that one set of RCAs split into two sets via a splitter cable, both your amps were being controlled by the single sub output control setting. If your head unit would have had two or more sets of RCA outputs, then you would not have needed the RCA splitter cable and each output would have had separate level controls.