The reason for this is that you always have to “log out” from the account.
As soon as everyone has been fully registered and the account was activated with the SMS PIN, it is also possible, for simpleness, if you are logged in with one account and then you can click on “Register a foreign sample”.
And then the CFP QR code of the other person or child has to be scanned and you can start with the test.
So you do not have to log out every time and switch from account to account.