for firefly, you can use masks, but it can be a little more complicated, depending on your shader. You would need to use a blender node for each of the sockets you use that have a different value for the 2 portions of the shader. Set the blending to 1 and plug your map into it.

For the physical surface and layers, set the transparency to 1 and plug your mask in it. White will be where the top layer shows.

And just for the fun of it, I made a quick shader with the physical surface node in the top layer and tried both the poser surface and then the cycles root nodes in the bottom and they worked too. so you can miss and match. But as I said this only works with the superfly render engine.

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