I don't know if TeamViewer will allow that, most desktop sharing apps will only work then one user is connected, including the Ubuntu bundled one.
FreeNX is a remote desktop server than can be useful for you. It will ask you for a username and password and open a new session with that user. It's tunnelled under ssh, quite fast and very secure. But it's not as near as easy to configure (read convenient) as TeamViewer: You will need to install the server, install a client on the computers where you are going to connect from, and open port 22 on your router. See this question and this.
Note that after the pain it works really well, we used it at work for a couple of years to let around 20 computers access our server through low-speed ADSL lines and we were very happy with it.