[brazen1]

Hi Warner.  That's a good thought.  To satisfy the OP that madVR is not responsible, we can simply eliminate it.  In MPC, change the preferred renderer from madVR to Default.  madVR will be out of the equation.  Your SBS.mkv should open in twin screens very briefly, your display should flash black as it goes into SBS or TAB, and a moment later it should display 3D with the two images overlapped.