Concat two MP4 files (side by side) with FFmpeg
Use

ffmpeg -i activity.mp4 -i mobile.mp4 -filter_complex " nullsrc=size=2560x720 [base];[0:v] setpts=PTS-STARTPTS, scale=1280x720 [upperleft]; [1:v] setpts=PTS-STARTPTS, scale=1280x720 [upperright]; [base][upperleft] overlay=shortest=1 [tmp1]; [tmp1][upperright] overlay=x=720" -c:v libx264 output.mp4


The shortest should be applied only to the first overlay.

Also, you can skip scale for the video which is already 1280x720. This output will only audio from one of the inputs.