It's quite simple.
I made two video tracks. Added the same video clip to both tracks.
Applied a mirror filter to V2.
Then I added a size and position filter to V2 using the top right preset. Set it up as per screenshot.
Applied a size and position filter to V1 using the top left preset.
These numbers are good for a video at 1280X720 resolution. They need to be adjusted for higher or lower resolution videos.
Also it's best to apply the mirror filter before the size and position filter.