Do you have both installed? I think you can choose which you want to boot in a desktop manager like kdm or gdm depending on what you use now or want to use now. I think you can simply change the session to the other in the login screen.
If you start from the commandline you can use:
"startx /usr/bin/startkde" and startx /usr/bin/startgnome" the last path might be a bit different but it starts with /usr/bin/start so your search is narrowed down to only a few options by then and then you can pick out the proper one yourself too