Is it possible to logout to any other DE without close any running application?
You can open n sessions with the same user :
you could install Xephyr o Xnest(older) and then open a new Xsession in a window (like a Picture In Picture):
startx /usr/bin/startkde -- /usr/bin/Xephyr :2
or
startx /usr/bin/gnome-session -- /usr/bin/Xephyr :2
or CTRL-ALT-F1, login and open a new X-window session:
starx /usr/bin/gnome-session -- :2
or any other session manager or command, ie xterm
And move between sessions with CTRL-ALT-F7,F8 Just beware of RAM :)