Good idea !
But then you have to demux the file twice again, because on this way you can only select ONE Audiotrack, and ONE Subtitletrack.
If you have more than one of each tracktypes you have to redo demuxing. Thats possible, of course, but not a fine or optimal way.
All a lot of work in the moment.
A nice suggestion would be to have a option in X1, or a seperate Tool for demuxing.
Must have a look if it is possible to demux with mkvextract and a modified chapterfile.
But very nice tool ! I like it, keep on developing please !