Recent 600 kms trip with 2 iphones in the car. One connected as a phone the second as the audio player. The Mark Levinson system was sounding tinny with frequent drop outs while listening to the music from the second iphone. The drop outs sounded like my old Rav 4 when the CD bounced around in the CD player.
Disconnected both phones and reconnected the 2nd iphone both as phone and audio player. No more drop outs and rich tones of ML back.
So, even though system allows you to choose to connect a phone and audio player separately via bluetooth, does not work practically.

 

For that long of a trip I would just jack the 2nd phone into the USB port.
Keeps the device charged and if you wait for the library index to transfer
you will be able to command song choices verbally. Press the speech/talk
button and say " Play artist Eric Clapton"...

 

Great advice. Thanks.