You probably want to combine the tracks using a mixer AU (like MatrixMixer or StereoMixer). Then send the output of that to a FileWriter unit (I think you'd want to set the offline rendering property here). If you need to hear it at the same time, things get trickier I think, because you can only have one output per graph. In that case you need to tap off another unit's buffers from the render callback, copying the data out of the callback function on each pass and manually writing the data to a file in a separate thread. I believe there's an example that does this somewhere (maybe the AULab app included with the Developer install).