I am not with Laminar Research; just a flight simmer.
You are totally correct in so far that XP10 and XP11 are totally different packages just like an apple is to an orange. Either package does not integrate (mix and match) with the other. The only way to upgrade from XP10 to XP11 is to purchase XP11 as a new product.
You need to be aware that XP11 is very resource hungry and may need to upgrade the specs of your PC. XP11 runs better on a windows based system using NVidia cards and Intel processor. Have a look at the specs found at https://www.x-plane.com/kb/x-plane-11-system-requirements/
Yes you can have XP10 and XP11 loaded on the same PC. What you need to ensure is XP10 files are in their own directory and not mixed through other directories. Laminar Research recommends that X-Plane be loaded to the desktop. However many serious flight simmers either create a directory on their " C" drive or have a dedicated drive just for XP.
Once XP10 is isolated then load XP11 onto your PC. Again load the XP11 software onto the desktop but better still load it onto the "C" drive in a totally different directory to XP10.
Provided you keep each package isolated from the other you should not have any problems with both packages on the same machine.