The other car has a preference selection that will try the preferred phone first and then will try whoever else is in range. I like this one best since I'm usually alone in this car. Either option really doesn't work great if you are sharing the car, the car would never really know who it should connect to if both phones are in range. I wish the car would somehow factor in RSSI into the auto connect decision, like don't pick the default or most recent phone if it has significantly less signal strength than the other devices in range.
That could show for 10 seconds or something and then pick whatever the default would have been.
In this case, I think it's a mix of both the car and the phone picking favorites, and my phone loses the toss every time because of some setting that I couldn't seem to find.
I believe at some point when we got our new phones, she accepted a dialog that set the car to a "trusted device" which means it will automatically turn on bluetooth and connect as soon as it's in range. But I couldn't find a way to turn that setting off.
Finally I had to force the car to forget both of our devices and I was able to get my phone to connect to the car while my wife was anywhere within a few feet of our garage.