Unfortunately, vehicles can currently only be registered to one QwikPark account as the app does not know who is driving at the time of entry and, therefore, it does not know what account to charge.
If you share a vehicle and a payment card, you can both download the app and use the same login / shared login details, enabling both users to be logged in on separate devices.
This way, you will both get the same notifications when a session starts and can both update and amend the account.
Please be aware that sharing login credentials gives the other person full access to your account, including payment methods and account settings.