And by that I mean the xfce that is provided by the endeavorOS installer, which is configured very different from the default one you can install (and much better imho).
I did it in the past without any problems. More complicated might be Gnome and Plasma installed both.
ah sorry, I meant, HOW do I do that?
No guarantees since I’ve never done that (gnome+xfce), but there’s an app named eos-packagelist that you can use to find packages to install for both gnome and xfce.
eos-packagelist XFCE4-Desktop
eos-packagelist GNOME-Desktop
So looks like you could simply install the xfce packages from the corresponding output, something like this:
sudo pacman -Syu --needed $(eos-packagelist XFCE4-Desktop)
But as said, no guarantees. One thing that might cause an issue is the display manager (gdm vs. lightdm), so it is (always) a good idea to have the USB installer around.
According to https://discovery.endeavouros.com/desktop-environments/how-to-install-desktop-environments-next-to-your-existing-ones/2021/03/ I simply would do this
sudo pacman -S xfce4
sudo pacman -S xfce4-goodies
But that does not give me the xfce setup from endeavour right? I really liked the setup from the boot medium with the task bar at the bottom and working hotkeys.
I will try that, let’s hope I dont kill my gnome 
I assume that endeavouros-skel-xfce4 3.7-1 has the EOS specific defaults. So this must be installed.
However, this might only work ok, if when logging in the first time after having installed xfce4 the ~/.config/xfce4 directory doesnt’ exist yet. Then it will be populated from what is found in /etc/skel/.config/xfce4
I see no reason why it wouldn’t work. But it may require some manual adjustments e.g. for settings. Hopefully all will go well.
That was installed when I tried, it still did not give me the “cool” config from the setup disk 
Just test by doing this
Log out from xfce, then go to a console with Alt-Ctrl-F5 or so and issue
mv ~/.config/xfce4 ~/.config/xfce4.save
Then go back to login screen and login.
When I tried it worked for me. For instance I got the panel at the bottom…
Or perhaps easier: create a test user and log in with the test user
I was able to at least enable the theme via the “welcome” screen from endeavorOS. The keyboard shortcuts are still not the same but I can manually edit them. Thanks everybody.
PS: removing the config for an existing user that had previously logged in will prevent that user from logging in.