Yeah. Luckily, this isn't a problem at all. If you automate the importing of the registry data from user account to user account it makes all this stuff match too. You're essentially constantly overwriting each user's individual registry settings with the settings that existed when the previous user logged out. It's actually even easy enough to prevent users from modifying any of the settings this way (simply don't have that account run the "save to REG file" logout script but still run the "import from REG file" login script).
Like I said... Not ideal (if the computer crashes before you log out it doesn't run the logout script and the settings aren't saved), but it could be worse. It would be much better for this to be an install option in MC12.1/13/whatever.
I can post the full versions as part of my Registry Saving Scripts if anyone's interested, with instructions on how to set it up properly in gpedit.msc.
I already did a couple of changes to the default file locations so that both user accounts can use the same database and thumbnails without any hassle. I don't need to synchronize the program options (at least for now). Actually, the users may prefer to have some of the options set differently.
Since I had this new PC available, I wanted to try how a vanilla MC12 can be used without any special tweaking and this was the first problem I encountered.
For most common users the only not too difficult workaround is to create a library outside the account specific folders. Though, MC would still create separate thumbnails for each user account and probably these would be constantly out of synch after other users' library edits.