Display Alternating Videos

Here's a solution:

• Place both videos off the slide.
• When the timeline starts:
  • Pause both videos.
  • Move one of the videos onto the slide with a very quick (e.g., 0.10 sec) motion animation based on the value of a toggling T/F variable, else move the other video. 
  • Toggle the T/F variable. (Toggling the variable will result in switching between the two videos.)
• Trigger each video to play when its corresponding motion path completes. 
  • Only one video will play, since only one motion path will complete, based on the variable.

You can see this in action in the attached file. (FYI: The videos don't overlap in this example. That was just for convenience.)