Shadow on Transparent Surface But Blocks Other Shadow Like Opaque Material

This might be asking too much.  I got shadow working on a transparent surface using a custom shader.  But I also need it to BLOCK shadow coming THROUGH so it doesn't appear on the surface behind.  Can I have my cake AND eat it?

This is for an AR Passthrough app.  There is an invisible virtual floor and wall that represents the real floor and wall, but the wall shadow is passing through to the other side of the wall as well as being displayed ON the wall.  I could make the floor end AT the wall, but I would still have the same problem on other objects like couch and desk that I'm trying to cast realistic shadow on.