From Apple Documentation on Fast User Switching:
AppleEvents have been enhanced to support Fast User Switching. Because of these changes it is now possible to send AppleEvents to applications running in any session, when the uid of the processes match. 'root', or uid 0 processes are allowed to send to any process in any session. Non-root processes can only target applications that match their uid.
To support remote AppleEvents in the multiple user scenarios, the typeApplicationURL definition has been enhanced with optional uid and pid parameters. Both uid and pid are optional. If not present, the active console user's applications will be targeted. As before, the username should be that of an admin user on the targeted machine.
So it looks like you need the UID and possibly the PID of the target application.