I don't think you can connect to the same machine as 2 different users - and windows will cache connections even if they aren't mapped to a drive letter. If it happens again, try 'NET USE' from a cmd window to see if you have lingering connections and delete them.
If it were a Windows server, yes, how ever if you use the netbios alias option in Samba, the Windows client thinks it is connecting to different servers, and the Samba server doesn't care. I know this works as I use it.
Brett