Just a couple of other thoughts in connection with the postings above--

It's not just outdated paper which can be used for back-to-back mounting. The enlargement which turns out to be less than a masterpiece still works great as mounting material. I don't discard the rejects, the full-sheet exposure test strips, etc. but keep them handy after processing for just that purpose.

When I do the standard mounting and matting, with either RC or fiber-based material, I use a reject or blank processed sheet on the back of the mount. The curl tendencies on each side of the board counteract each other and virtually guarantee flatness. The only drawback is the cost of another sheet of mounting material and a little extra time.

I've never had occasion to do back-to-back with fiber-based paper, but it should be just as practical as with RC.