two different gnome-software stores - same os Xubuntu 20.04lts - starting snap-store starts gnome-software?

Solution 1:

From what I have read before - the Ubuntu Software is a snap application, it does not have integration with FlatPak.

So you may to decide removing it:

snap remove snap-store

and install deb-packaged GNOME Software with FlatPak and Snap plugins instead:

sudo apt-get install gnome-software gnome-software-plugin-flatpak gnome-software-plugin-snap