physics.
but perhaps you’re a wedding photographer, not a model photographer, simple, use a black on both sides if you’re shooting the bridal photo of the bride in a white wedding gown and if it’s a photo of the bride and groom side-by-side, and your groom sports the traditional black tuxedo and the bride her white wedding gown, you’d have a black screen on the bride’s side of the frame and a white screen on the groom’s side of the frame.
if the model were wearing a white wedding dress, we’d change the left -2/3 stop white screen with a black fabric screen with this set-up.
you’d use this same technique in the 