Ok, great.
I've been sitting here pondering this. Whether to assemble the design in colour first, then make it all black-and-white afterward; or to edit the photos separately in black-and-white, then assemble the design.
Since there might be some blend mode work involved, which is hard to apply to multiple layers at once, I think we should go for the latter.
So go ahead and edit both photos in black-and-white in the usual way, then post them here for me and we'll discuss combining.