split a video in 2/more parts

mencoder Aliens\ 1986\ Special\ Edition.mp4 -o alien2-1-mp3.avi -endpos 4633 -ovc copy -oac lavc
mencoder Aliens\ 1986\ Special\ Edition.mp4 -o alien2-2-mp3.avi -ss 4620 -ovc copy -oac lavc

the offset between endpos and ss is to avoid missing frames for not finishing the video on a sync-frame