cocoa Mixing audio from two microphines

I'm trying trying to find solution for mixing audio from two mics. For now I have 2 video avcapturesession with one audio (input and output) and one video(input and output) for each one. How I can mix audio two cmsamplebufferref to one and after that send mixed cmsamplebufferref to file writing? Or maybe I can add two different audio inputs to one avcapturesession and got mixed cmsamplebufferref from one output? Any ideas?

Thank you for any advice.