iOS app submission - access to username/password
App Store Review Guidelines necessitates this. You will be rejected if the app requires login and you do not provide credentials.

Before you submit:

Provide an active demo account and login information, plus any other hardware or resources that might be needed to review your app (e.g. login credentials or a sample QR code).




2.1 App Completeness
… Make sure your app has been tested on-device for bugs and stability before you submit it, and include demo account info (and turn on your back-end service!) if your app includes a login. …

Also, how does your security team ensure that your first two users won’t see each other’s data? If adding a third account for Apple breaks your security, you’ve got your research can be concluded with the answer: the back end team is not ready to ship.