How to run 2 X sessions with different display managers?

Questions #1: How to run multiple X sessions at the same time

You need to run sudo startx gnome-session -- :1 vt8 - the vt8 tells X to start the session on TTY8 (which you get to with ctrlaltF8)

Question #2: Drag windows between X Sessions

I doubt that's possible, and if it is, with the software we have today it would be very unstable - it's the job of the DE to do everything from window management and toolbar rendering to workspace placement, and you can't just 'cut and paste' a window from one DE to another. In the future this could be possible, as it sounds like something the Wayland graphics server (hoped to be a replacement for X) could be better at managing. But for the moment at least, it's unlikely to work.

I am using 2 different sessions on Ubuntu like this:

• Create new user
• CTRL+ALT+L (switch account)
• Log into using that user

Ubuntu creates new X session in next tty when another user logins. So now you can switch between those sessions using CTRL+ALT+F7 and CTRL+ALT+F8.

I've never tried with Gnome3 but it works with XBMC session