How to use one Android phone as a handsfree device to another (via Bluetooth)?

I have an extra Android device (a mobile phone, Samsung Galaxy Y Duos, S6102B, Android 2.3.6 - rooted), that I call D1 here, and would like to use it as my "car brain", like a low-cost multimedia central, to do a lot of things. On this question, I would like to know: * If I have another Android device, called here D2; * How to play every sound from D2 on D1 using bluetooth connection. Example: 1. I'm on a call on D2. When I enter the car and turn D1 on, when D1 and D2 become bluetooth-paired, I can drive and continue talking via D1 (like a handsfree device), with D2 on my pocket; 2. I'm hearing music on D2 with my headphones. When I enter the car and turn D1 on, when D1 and D2 become bluetooth-paired, I can hear the music from D2.