In a separate console screen (CTRL - ALT - F1 to F6 ) You can login on the command prompt. When you're logged in, you can give the startx command to start a X windows session.
Switch to another console mode by pressing CTRL+ALT+F1. Provide your login details and type the following command to start X session:
startx -- :1
The default X session runs on screen 0. However, you can run the more X sessions on screen 1, screen 2, and so on. The above startx command tell X to run on screen 1. To run X on screen 2, you'd use the command
startx -- :2
Please note that CTRL - ALT - F7 to F12 identify six X sessions.