Strange. Unix/Linux systems do not “boot into a desktop”. They boot and when they are set to boot into runlevel 3/graphical, they will have a login screen. Period.
From the login screen a user can chose to login into the DE of his/her choice (if more then one DE is available). But that is not boot. It is login. And the user can logout and login again, and more users can login, even at the same time. All without the system having to boot.
And when you want to install a DE, use YaST > Software > Software Management and choose for the Patterns Tab. Scroll down at left and there are patterns for several DEs. Select them and install.