< How to start X sessions >

There are two ways to start the first X session: you either start the X Window System manually after logging in, or X starts automatically when your Linux system boots up. If your system is configured to start X automatically, you don't have to worry about the first X session: it's already running.

If you don't have a graphical login, you probably start X with the startx command after logging in:
startx

The first X session you start runs on screen 0. It does this by default, so when you start an X session, you don't have to specifically tell it to run on screen 0. However, you can run the second X session on screen 1, the third on screen 2, and so on. This is how you tell X to run on screen 1:
startx -- :1

Of course, to run X on screen 2, you'd use the command startx -- :2, and so on.

< Switching between X sessions >

You probably know you have several virtual terminals. On a default Linux configuration, you have command line sessions running on your first six virtual terminals. Your first X session is running on the seventh virtual terminal (screen 0). If you're running only one X session, all the terminals after terminal seven are empty.

As you probably guessed, the second X session runs in virtual terminal eight, the third session in virtual terminal nine, and so on. You switch between X screens the same way you switch between virtual terminals: Press Ctrl, Alt and the F key with the desired terminal's number.

For example, to switch from screen 0 to screen 1 (from the first X session to the second one), you'd press Ctrl + Alt + F8. To go back to the first X session, you use Ctrl + Alt + F7.

< Useful tips >

When starting multiple X sessions with startx, make sure you have a file called .xinitrc in your home directory. It's the file that controls things like what window manager is started. For more info about .xinitrc, have a look at the Changing the default window manager tuXfile.

Because the default screen is 0, some graphical applications may get a bit confused when using other screens. If you type an application's name at the command line of a terminal emulator, the application may run on screen 0 although you launch it from another screen. This isn't a problem, though. Many applications have a command line option for specifying the screen it runs on. For example, to run Gimp on screen 2, you'd start it with:
gimp --display :2

This is actually an advantage. You can launch the application from any X session or virtual terminal you want and send it to any X screen you like!

No one is forcing you to use the same window manager or configuration in all the X sessions. This is probably one of the reasons you'd want to run multiple X sessions at the same time: to be able to quickly switch between different window managers, resolutions, or color depths.

To have an X session with another color depth than the default one, you'd use the -depth option. For example, to run a second X session with a really ugly 8 bpp color depth, you'd type:
startx -- :1 -depth 8

Of course there are much more options than just the -depth option. To get more help with startx, check out the manual page:
man startx