Can Multiple Users open an app in Power Apps simultaneously?
Yes, In Power Apps, the app can be opened and used at the same time by any user with whom you have shared the application.
Can Multiple Users edit an app in Power Apps simultaneously?
No, In Power Apps, only one user can edit the same app at the same time,
Also, if the same user has tried to edit the same app that already opened before in another session, The Power Apps will show a warning message to close the other sessions or override.
You already have a blank app locked for editing in another session
Close your other session to continue, or override it with a new session (you could lose unsaved changes).