A curious question, in APTus exists the option of change to another graphic environment ( as for example of KDE to MATE) the add of MATE (for example) its removes automatically the previous KDE? or left them - the 2 as option to the enter to the system? And i can select one of them each time want to log in?
One potential problem is that some of your dot files might conflict between the two (or more) DE's. Or they may not. I have only used "tasksel". "man tasksel'
It (and aptus I would believe) should automatically set up lightdm or sddm, etc. to allow you to choose at boot which DE you want to run.
APTus Desktop item install the desktop environment or window manager.
It does not delete desktop(s) that already exist.
You can choose those desktops when you log in.
In the case of default (LightDM), it is probably the icon on the upper right.
Note: KDE Plasma 5 is displayed as "Plasma" in the session item. (not "KDE")
Don't forget that if you install alternative desktop environment you will have tons of other applications installed.
It would be better to to test those in virtual machine.