How do I use one black and white image as a mask for another image in GIMP?

Question

I have an image that requires a really fancy mask, created through multiple selections and subtractions of the existing image. Here's a concrete example:

Layer 1: original image, transformed into a black and white mask 
         (would be difficult to do this with "selection to mask"

Layer 2: the original image, with an image mask that is made up of the black and white
         image in layer 2 (this can be a one-time copy of layer 1, and does not need
         to be transferrable back to Layer 1.

Hope that's clear! Thanks!

Answer 1

Can't you use the Add Layer Mask option and select Grayscale copy of layer?