In parallel, not, but have both graphical env., for choose in the session, in the login screen, is normal and possible.
Example, with Sulfur, install GNOME in a previous Sabayon KDE installation.
I have one installation with all graphical env.: KDE, GNOME, XFCE, E17, LXDE etc...
ALBERTO FEDERMAN NETO
Favorite Distros: Sabayon, Gentoo, Mageia and OpenSUSE