Can one exchange SSD's in a single MBP for doing a back-up (delayed) on one of them?
Apple recommends backing up your HD as they "can't guarantee the integrity of the data on hard drive." Or words to that effect.

Theoretically you can do what you have proposed. You may have boot issues, EG the Mac the SSD is swapped into may not boot but that is unlikely to damage either the Mac or the drive, so if you have the Mac and the time, go for it.

Personally I'd put the SSD in an external case, plug it and your backup drive into the same Mac and make a bootable image file of your SSD onto the backup drive. It's a little easier and you are less likely to damage more than one Mac accidentally by digging around in it... IMHO...