You want to run two apps simultaneously on the same port? You’d have to put them on the same flask server with different prefixes, or put another server in front. There have been some discussions about that like Hosting multiple Dash apps with uWSGI + Nginx (but note the API has evolved since then)
If you mean you’re done with one app and want to run another on the same port, I’ve only seen that mean the previous app is still running - you just have to ensure it’s really stopped before starting the second one.
Happy to help sort it out if you can give some more details about the symptoms you’re seeing.