In my home directory I have the following files:
.xinitrc
.xinitrc.kde
.xinitrc.gnome
When I type startx, blackbox starts up (vt7 on display :0)
I then have a script called xkde that looks like:
#!/bin/sh
startx /home/ang/.xinitrc.kde -- :1 vt8
Another called xgnome:
#!/bin/sh
startx /home/ang/.xinitrc.gnome -- :2 vt9
I can then open up an xterm, run xkde and xgnome and I can have blackbox, gnome, and kde all running at the same time.
I can then use ctrl-alt-[f7|f8|f9] to switch between them