Knowing what the original glue is would be important. Some can be reworked with various solvents, but those same solvents may affect the ink a card would be printed with, and/or the blue used to stick the layers together is they used a layered stock.
I have a few old postcards that I'm pretty sure got very wet for a long time, and are now a front and back.
Testing a small amount on a small area, or on something that's come entirely loose would be very helpful.
Water
Solvents like Those already mentioned plus mineral spirits or lighter fluid
Alcohol - some old glues may be alcohol based and similar to shellac. Those will readily soften with nice clean alcohol applied.
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