To do the test disc I sent to Mitch I used some re-authoring program (forget which) that allowed me to just strip out a load of chapters, leaving the video & audio intact (plus menus if there were any), so using this program it should be possible to spit out two almost identical SL DVDs from the DL DVD, one with the first half of the film and the other with the 2nd half plus the extras and nothing re-encoded or dropped to save space.