At delivery, we had to use the original account on both phones. (I've seen reports that the delivery clerk can promptly enable a second account to access the same VIN, but our guy was a little scattered, so we went with same account at first.) A day or three later, I emailed servicehelpna to add the VIN to my wife's account. Took about 17 days for them to reply that they had done so.
Now, she signs in with her account, and has equal control access to that car. Only visible difference is that she doesn't have a loot box.