the trick is to put them face to face and back to back
(between masonite or clean boards)
when the prints aren't tacky to the touch, so they won't glue themselves together.
i use a nipping press and i use that from time to time,
and i also have a book press i got from pottery barn ( yes its true! )
and a small one i got from gaylord brothers.
for big things, i back to back face to face them, and put them between sheets of matboard and under countertop.
a bookbinder i worked with used to have heavy glass block ( ww2 navy warplane ) windshield. it worked great! he wasn't making photographs flat, but things he pasted/glued. kind of sort of the same thing...