I have two Android phones which I wanted to set up one as the one that answers calls for the second that could stay in the backpack. Is it possible to use a Android phone as a Bleutooth accesory, like a hands-free device?
The alternative solution that comes closest to what you're after is to install mysms mirror, which is an app that tells you when your other phone receives a call or text message, and lets you call back. See their blog.
Google Voice might also work for you. It's mentioned in the answer to this question: Answer my phone on my Tablet?