measurement - How do I use directional couplers on a differential signal?

I understand how a directional coupler can be used to measure signals travelling in one direction on a wire. What I don't understand is exactly how this would work on a differential signal, E.G. LVDS (which stands for Low Voltage Differential Signal).

My ultimate goal is to test the quality (impedance really) of hand made cables. Ideally, they need to be 100R impedance. One way to test the quality is to measure the amount of reflected signal from the terminator.

Measuring reflection using directional couplers is often done for single ended RF signals, but I have never seen it done on differential signals.

Do I do it like this, using two couplers (these ones), one for each wire? If so, which two output ports should I watch if I want to see reflections from the terminator? A&C, B&C, A&D or B&C? Should I add the signals from the ports, or subtract them?

Alternatively, is there such a thing as a differential directional coupler? I tried googling for it, but nothing much came up.