Why do you need to have two package formats that do the same thing installed by default? If you could install snaps and flatpaks both from the same store you could have 2 (or 3 if you also installed the .deb) copies of the same app, like steam etc installed, and user sessions and games set up on one wouldn’t be launchable from the other because they all store their state and config in different locations - the only way to know what config your program is launching with would be to inspect and rename the launcher scripts. If you are intending to support naive users this is the absolute worst case scenario. It would be like debian including pacman by default as well alongside apt for maximum user accessibility confusion.