I'm going to assume that you mean logged in at once, and not one "login" at the same time.
I've never written a Django application before. But one method I've used in other languages, is to store the session ID of the logged in user in their user row in the database.
For example, if you have a users table in your database, add a field "session_id" and then when the user logs in, set that to their current session_id. On every page load check to see if their current session matches the session_id in the users table. Remember whenever you regenerate their session_id in your application, you'll need to update the database so they don't get logged out.
Some people when a user logs in, just store all the users details into a session and never re-call the database on a new page load. So for some this "extra" SQL query might seem wrong. For me, I always do a new query on each page load to re-authenticate the user and make sure their account wasn't removed/suspended and to make sure their username/password combo is still the same. (What if someone from another location changed the password, or an administrator?)
