How can I mask an object to allow another object to show through? I have an outer eye and an inner eye.  I'd like the blue iris to show through the outer eye.  In my source files shown below, I've got a couple masks.

I suspect the answer is an image texture node with the mask file, but I'm not sure how to set that up.

If another solution seems better, I'm open to suggestions.

For reference, the .blend is here.
Thanks!
Generally, you would combine the 2 images using a mix shader, and use the mask image as the mix factor. See Below: