You’re not the only member asking this recently.
No there’s no ‘easy’ method, but I will add a new option to the upcoming MMH 9 beta release in the Channel Remix tool. That will allow a one-click batch remix of all 8 channel FLAC or Wav files to stereo.
You can also do it now by using the Channel Remix tool’s ‘User Defined’ option and create your own channel mapping 8 inputs and stereo output. You can set the volume balance by editing the channel gain value in the user defined diagram. See the PDF docs page 109 onwards. I would recommend using higher gain for source FL & FR and less for the 4 surrounds, it will be remix and listen (rinse and repeat) on first file until you get a best result, then use that for all your remixes.
The new MMH version will probably first remix 4 surrounds to 2 (making a quad) then quad to stereo in an attempt to balance the final remix. That process would be invisible to the user.