I read about the virtual terminals a little and that gave me an idea. I searched for a way to have two X sessions simultaneously.

I had a look at these sites :

[1. How to run multiple user X sessions on the same computer, at the same time?](https://askubuntu.com/questions/14873/how-to-run-multiple-user-x-sessions-on-the-same-computer-at-the-same-time)  

[2. How to drag windows between 2 X servers?](https://askubuntu.com/questions/74509/how-to-drag-windows-between-2-x-servers)

I tried `startx -- :1` but my earlier terminal (on `Ctrl`+`Alt`+`F7`) hung up. How do I ensure this does not happen?

My main need is to get Unity on `:0` and Gnome-shell on `:1` and switch between them like workspaces.

If possible, I'd like to get login screen on tty8 or get option of logging into a different account.

As per [question 2](https://askubuntu.com/questions/74509/how-to-drag-windows-between-2-x-servers), exchanging windows between sessions is not recommended as it is not easy. But if it is possible I'd like to know.

I have Ubuntu 12.04 64bit.

The above command made both sessions crashed (virtual one worked for 15 min then crashed when I switched to real).

Also, how to revert to one X session if system gets overloaded?

Update :
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Tried code in answer 3 times but system never stabilizes. Either tty7 or tty8 works, but not simultaneously, for sometime before crashing.