Make 3 layers.
The topmost layer is your mask layer.
The next two layers are your "white" layer and your "black" layer.
Set the compositing mode for the mask layer to "Multiply"
Right-click on the mask layer, and look for "Mask Mode", set that to "One Below"
You should now be able to trigger clips on all three layers, and the top layer will act as a mask, where the white parts will reveal the 2nd layer (the one below), and the black parts will reveal the lowermost layer.
Way easier with R6 than with R5.
I'm trying to achieve exactly this in Avenue 6 however i'm very new to this and don't quite understand what you've done in that latest picture. Do you have some terms you could reference that I could google or a link to the part of the manual that's relevant? Or just maybe an explanation of what you've done? Sorry to be of trouble and thanks in advance