It is absolutely imperative that you DO NOT USE Selective Sync on either user while running in such a configuration. Switch to the other user and install Dropbox under that user, specifying the same location for the Dropbox folder. The basics of it would be to install Dropbox under the first user and specifiy a common location for the Dropbox folder (a location that both user accounts have full rights to). It's possible, but you're on your own to get it working as it is not a supported configuration.
