You can do that, but it is pointless. The original console will just format the drive that was formatted in the other console again. No PS3 will use a hard disk that was formatted in another PS3, without formatting it again. If you want to format one of the drives, then just format it in the console that it is already installed in. Swapping drives is a waste of time.
Correct. Each PS3 encrypts the data on the hard drives that it formats with a unique key. The PS3 can recognize the drives that it formatted, provided that the data isn't corrupted, and will leave those alone. Any drive with a file system that the PS3 can't read will result in a formatting prompt before the PS3 will boot.