To echo everyone else's answers it's possible but has problems even for an all windows environment.
serverfault.com/questions/298418/roaming...-logins-of-same-user
On a samba3 setup I have simultaneous logins are allowed but I've redirected as many folders as I can so that the write last conflicts are only likely to happen to appdata\roaming and the registry hives (like ntuser.dat). But.... Those are the files that cause problems and I've had corrupted profiles in the past because of the write conflicts on logout.
I've seen simultaneous logins work before at a university but they were using mandatory profiles (read only profile data) and redirected everything else.