multiple audio sources

I am looking for something that will allow me to play audio out of more than one audio source at the same time. Before I get tons of "why", let me do it this way. I connect the phone to my car with bluetooth. I want to be able to play music either on internet radio or stored mp3 while also listening to another streaming audio. I can't find anything that will do this. i can find stuff to play multiple mp3 or play to multiple speakers but nothing that takes audio from different apps and allows them to play at the same time.