If you want to auto-choose the next video, use either the Instant Switch or Seamless switching mode. The auto-chosen video will be the first video you connected to the clip, and will be visible at the top of the overlays list when you are in the overlay editor. 

If you need to change the default choice on an existing clip, you need to remove and re-add the connections so that the desired choice is at the top. If you want the video to pause at the end of a clip choose the Wait mode.