Shift movie horizontally for side by side
I'm looking for an answer that uses just iMovie or QuickTime editing feature but until there is an alternative provided, I will post this answer using the free VideoPad app (https://itunes.apple.com/us/app/videopad-video-editor-free/id846931335).

VideoPad has a Position Video Effect that allows me to slide the video to the right (the left area is filled in black).  Saving this I can now position it side by side and have the action appear on the right half back in iMovie.

Support for positioning video within the frame is what I need in iMovie