If they are truly valuable (i.e., you're willing to pay real $$), contact an art restoration expert, perferably with experience in photographic preservation, and pay them.
Otherwise, try the steaming first. If not successful, the distilled water soak with gentle teasing will separate the papers. However, you will probably find the emulsion softer after it dries than previously. If the photos are kind of valuable (not enough to pay lots, but still...) you could always make some stuck photos to practice on. Wet photos, press 'em together and dry, like in a low oven or under a heat lamp. Keep 'em pressed while drying.
These are just opinions - I separated stuck two papers once. I wetted the back of one of 'em (the exposed paper side), and kept wetting it until I could tease the papers apart. It was slow and tedious. It worked OK, although there was some transfer of fibers onto the contacted emulsion. If they had been emulsion-to-emulsion, it probably would have destroyed both.
"Keeping shooting will do more for the excellence of your photography than most any camera will!" JimmyD, PGS Extraordinaire.