Can I use Echo Auto with multiple phones?
Yes, the Echo Auto will connect to six different phones, although you might have to manually pair with additional phones once you've initially set it up. That's a simple case of pressing and holding the action button on the top until the orange light comes on, then it's in setup mode again.
Using the Alexa app on the second phone you can then run through the setup process again.
That's fine when there's only one of those phones in the car, but if multiple users are in the car at the same time, you might have to manually select the phone it's connecting to, by turning off Bluetooth on devices you don't want it connected to.
This only really matters when it comes to ensuring it's drawing data from the right device or accessing the correct contacts list and so on.
https://www.pocket-lint.com/cars/news/amazon/152634-how-to-setup-your-amazon-echo-auto/