I currently have 13.2 Gnome installed on my system and would like to also install 13.2 Xfce. Is this possible on a UEFI based system?
TIA!
Note: I am also running 12.2 on this system but had to manually set it up to use a different directory than the standard “opensuse” in the EFI partition.
Just do the same in this case as well. Like openSUSE_Gnome and openSUSE_XFCE.
Is there any special reason you want separate installs for the two different desktops? You can install both desktops (or even more, for example also KDE) in one installation. You simply choose which desktop to use, before you log in at the login screen. Click on the wrench on the login screen, a popup will show, select the Desktop environment you want to use, then log in.
You do not need separate installations to do this. 
I’m curious as to this also, albeit in my case its more for a simultaneous 13.1 and 13.2 (and possible later a 13.2 baseline and a 13.2 tumbleweed).
How does one instruct the YaST installer that the /boot/EFI for the 13.3 openSUSE is to be treated different from the 13.1 variant ?
You can do this with the “Distributor” setting in Yast boot loader. For 13.2, I used the default “openSUSE”. In a separate install, I used “factory”. The name of the directory used in the EFI partition is the Distributor (or the first word of the Distributor).
I’ll need to go through a typical installation again (or flash up boot loader in YaST), as the terms “Distributor” and “openSUSE” in this context (of the YaST boot loader) is a bit lost on me.
Honestly, I was thinking of 13.2 and not 13.3 above, although if there will be a 13.3 (or 14.1 as appropriate) I assume the question is still relevant.
Just run Yast bootloader from within your system. You can cancel out, to avoid making actual changes. You should see the “Distributor” line on the opening screen (left tab). It just sets the “GRUB_DISTRIBUTOR” line in “/etc/default/grub”, and the installer uses that to determine the name of the directory in the EFI partition.