The secondary user of my hubs is configured as a "Guest", and notifications using the app seem to work ok. If having two Admins fails again (no reason it should), consider setting the second account as a Guest account.
Edit - I remember having notification issues with the app when adding a secondary user. I don't ever remember the secondary user being an "admin". So I stopped using the app for Notifications and use email-to-sms to receive all notifications as SMS.
