Unless I am confusing your question, the only way that I know how to extract the Multichannel files from the ISO file is by using the ISO2DSD process and selecting "Multi" instead of "Dual". After I do the extract to create the .dsf files, (and modify the tags, add a folder.jpg, etc.) I use dbpoweramp to create 88.2 resolution FLAC files to save to another USB drive to play on another system that I have at a relative's house which can't process .dsf or dsd over pcm (dop). I do this for each of the stereo and multichannel SACD sections. My laptop is old and slow, so my SACD ripping/archiving process is moving along very slowly!