I am looking for a command line tool to join 2 video files, however I want the videos joined split screen frame by frame instead of one after another.
Any ideas? Seems this is not possible with ffmpeg.
With a recent version of ffmpeg (assuming both videos are the same resolution):
This essentially doubles the size of input1.mp4 by padding the right side with black the same size as the original video, and then places input2.mp4 over the top of that black area with the overlay filter.
What is the source format of your video? If it is AVCHD files in .mts or .m2ts, you can use tsmuxer command line version to join clips. You can get .ts or .m2ts as final format.
I am not sure what you mean by saying you want the videos joined split screen frame by frame.