Duplicating 2 apps and having them point to the same new sheet is not possible at this. Also, changing the sheet on an existing app is not possible. I’m not a big advocate of having separate apps for Admin and Users. I realize that it may be necessary in some situations, but more and more features are allowing for the ability to create Admin sections or Admin edit rights to an app intended for Users. I think it comes down to there being a lot of risk if you change an entire spreadsheet if something doesn’t match exactly. I’m not saying it shouldn’t be a goal at some point, but for now it’s a limitation.
I would offer the suggestion of creating a template of one app with both the Admin and User tabs, When duplicating, you would first duplicate with a new sheet. Then duplicate the new app with the same sheet. Then hide/remove tabs/components to make each one function as an Admin or User app.