I'm not quite sure I understood your question correctly but if you ask how to tell "startx" to choose between gnome and xfce when you want to change from one to the other, then you can do (as normal user) "nano .xinitrc" and make it look like this (and then exit nano saving changes: Ctrl-x, y, Enter):
Code:
#exec startxfce4
exec gnome-session
This will tell "startx" to start gnome. When you want to start xfce, do "nano .xinitrc" again and make it look like this:
Code:
exec startxfce4
#exec gnome-session