You'd be better off building two apps. If you think you'd be saving on work by building the two endpoints in one app, you'd be mistaken, because you can't share types between the two endpoints. GraphQL would tell you it won't work.
You'd also be better off with two different apps from a scaling point of view. If your shop app gets a lot more traffic, you'd want to be able to scale it out and not the admin app with it, right?