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 (we are working on options for this now).
If you do share a vehicle and also share a payment card, then you can both download a copy of the app and use the same login / shared login details, which will enable 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 etc.