Choosing audio from multitrack mxf master.

I need to make H264 HD & SD stereo versions from my program master that has 12 Channel split track audio created in AVID.  I want to just use the first to channels of the master which is the stereo mix.  I know that choosing the Quicktime format allows you to choose the source channels 1-2, but the H264 format doesn't give that audio option.  I've tested the export to H264 and it sounds like it's exporting just the stereo mix rather than a mixdown of all the tracks.  Is there any way to confirm this? Or is there a way to edit the source tracks from the master file in AME? I know I could export a Stereo version of the master from avid, but I'm trying to avoid the extra step.  I'm currently working with AME 2019.  Any knowlege or advice would be greatly appreciated.  Thanks!