Even if you think you can take these apart and put them back together and still have them work, we don't know that the internal battery is any standard coin cell. If it was, the ability to externally change it would likely have been part of the design. It could be something that's not standard, a soldered-on part of the camera's circuitry, or even a capacitor rather than a battery.
If you're adventurous, open up one of the cameras to explore. Save the other one. You don't need the right date and time for full function. You just won't have correct file dates.
