Just have both users use the same catalog!!! Move the catalog file to a shared folder on the ROOT of the Macintosh HD (Set permissions so both users have read+write permissions). The actual program files should be usable by both users as the Applications folder is on the ROOT of the drive and accessible by both users. You may have to create a alias to it on the second users desktop or in the Dock.
Store all images file in another folder on the ROOT of the Macintosh HD or on a always connected external drive and you should be good to go.