I had the same issue.
I imagine you used the same password for both you and your dad.
Eg joe bloggs@internet. Password Dad
So when you log in it will go to the first account you set up.
But will not go to the second account with the same login details
What I had to do was go into the account that came up and then change the password on that account.
Eg joe bloggd@internet. Password son
Then you can login to both accounts using the same email address but different passwords.
You will need to get into both accounts as your dad will get an email asking him to accept the managing of tickets by you. (Or the other way round if you have used your dads account - being the only one you can access at the moment).
Hope that makes some sense.
I had the same issue, I got round this by having 2 separate passwords for the same email address. For my son and myself. That way we can log into both accounts independently. Also both are linked via My network.
Thanks guys. All sorted. It’s probably helped others too unless I’m the only technophobe