That is true. I've used screen -x to attach to screen sessions in multiple locations simultaneously for a number of years, and you don't need to do anything else for it to work.

On the other hand, multiuser mode CAN be used to let two DIFFERENT users to access the same screen (so you don't have to share a login password or allow an SSH key with essentially full access to the account).

By default, this is not allowed on Debian, even if you turn multiuser mode on, but if you absolutely need this functionality, you can do it this way (I just tested it---I, of course, don't use it this way):

First, make sure, as root:

1. Your /usr/bin/screen is setuid root. You can do this by running:
chmod u+s /usr/bin/screen
but this has quite a significant security implication, which is why this is not Debian's default setting.

2. Your /var/run/screen should have 755 permission (again, this is not Debian's setting (775 permission), as Debian lets setgid and group ownership of screen executable by utmp to handle writing to this system dir). You can do this by running:
chmod 755 /var/run/screen

Second, as the screen-sharing user (say, "debian"):

1. Run the "^A:multiuser on" command as described above.

2. Add the user to share it with (in this case, "test") by "^A:acladd test TMHNdoMbEaNfU". The last argument is the crypted password (you can generate one with mkpasswd, if necessary. If you were wondering, that's the crypt hash of "blah").

Now, as "test":

1. Run "screen -x debian/5321" where "5321" is the screen session ID (or you can type the first few digits that will uniquely match one screen session). You will be asked for password, type it, then now you have two users sharing a screen.

...

But, in my opinion, this is such a complicated setup, and unless you need the ACL features of screen (more detail at: http://aperiodic.net/screen/multiuser), the gaping hole that suid root of screen executable leaves is hardly worth it.