If I get what you want to achieve ...I don't see how retaining the OEM proportioning valve will help.

Your "low brake fluid" switch should be on your master cylinder ..not your proportioning valve. You proportioning valve should have a "hydraulic imbalance" switch that will tell you if you have a failure in either fluid circuit.

I suppose (although I really haven't thought it through a whole bunch) if you got a couple of "T" blocks you could rig them in PARALLEL and it would work.. You would just choke the adjustable until you got the right balance ..and your light system (which ever one you are actually referring to ) will remain totally intact.