I have a multiseat setup. 

On the X :1 server xbmc (a home media application) is running. On the X :0 server, kde is running with a different login. If the :1 X server started first then xbmc can mount/unmount media, if :0 X started first then kde can mount/unmount media. In each case the other program (and login) get "Unauthorized access" error. 

Here is the poutput of ck-list-sessions
$ ck-list-sessions 
Session4:
unix-user = '1000'
realname = 'Lorant Farkas'
seat = 'Seat1'
session-type = ''
active = FALSE
x11-display = ':0'
x11-display-device = '/dev/tty8'
display-device = ''
remote-host-name = ''
is-local = TRUE
on-since = '2011-08-31T19:50:03.165447Z'
login-session-id = '4294967295'
Session1:
unix-user = '1003'
realname = 'XBMC user'
seat = 'Seat1'
session-type = ''
active = FALSE
x11-display = ''
x11-display-device = ''
display-device = '/dev/tty1'
remote-host-name = ''
is-local = TRUE
on-since = '2011-08-31T19:49:12.603486Z'
login-session-id = '1'
idle-since-hint = '2011-08-31T19:49:43.153517Z'
Session2: 
unix-user = '1003' 
realname = 'XBMC user' 
seat = 'Seat1' 
session-type = '' 
active = TRUE 
x11-display = ':1' 
x11-display-device = '/dev/tty7' 
display-device = '/dev/tty1' 
remote-host-name = '' 
is-local = TRUE 
on-since = '2011-08-31T19:49:17.092264Z' 
login-session-id = '1' 
Session5: 
unix-user = '1000' 
realname = 'Lorant Farkas' 
seat = 'Seat3' 
session-type = '' 
active = FALSE 
x11-display = ':0' 
x11-display-device = '' 
display-device = '' 
remote-host-name = '' 
is-local = TRUE 
on-since = '2011-08-31T19:50:03.325718Z' 
login-session-id = '4294967295'

As I see the problem is this: at any time only one user can be active.

On a unix-like system theoretically numerous user can be active in the same time. Both XBMC and KDE should access all possible media.