There might be an easier wasy, but this is how I do it:
This would get the first half of a movie. Go to where you want to split. Do an Edit / Set Selection Start (also one of the buttons at the bottom). Then go to the end of the movie and do an Edit / Set Selection End (also button). This will hightlight the selected part blue. Hit the delete button to remove the selected section. Now make sure that both video and audio are set to Direct Stream Copy. Now just do a save and let it go.
Good Luck,
ERJ