I agree with dpollitt, the parts of the photo stuck to the glass are most likely damaged beyond salvage. Although I've seen miracles come from someone who is well skilled in Photoshop!
That said, to separate the two from each other, I would first try using PEC-12 Photo Emulsion Cleaner.
You may also want to try and immersing the whole lot in a pan of distilled water. Using distilled water to prevent any damage from dissolved minerals. This may re-hydrate the stuck parts, and, as long as the print is not water soluble ink, just dry it quickly after and wipe it down with something like PEC-12.
If that doesn't work, as a last resort, I can only suggest using regular lighter fluid. I learned this trick from being a photo Grip for a catalog shop many years ago and would use lighter fluid to clean things like tape residue from all sorts of really expensive pieces of furniture and glass. I've also used it very successfully on other things like plastics and porcelain. All without any damage to anything.
If I had to guess, the slippery properties, and lighter fluid's ability to loosen things adhered to others come from it being petroleum based. But as mentioned, I would only do this as a last resort as I've never before applied it to a photograph.