How to join two video frames into one, so the final video plays two frames as one simultenously

If both videos have the same height, you can use hstack you can also do it 'vertically' with vstack, above Vfilter is faster than pad + overlay to produce same output:

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex 
'[0:v][1:v]hstack[vid]' 
-map [vid] -c:v libx264 -crf 22 -preset veryfast output.mp4

This can be done with ffmpeg. If the videos are the same size:

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex 
'[0:v]pad=iw:ih*2:0:0[intv];[intv][1:v]overlay=0:H/2[vid]' 
-map [vid] -c:v libx264 -crf 22 -preset veryfast output.mp4

If they're different sizes (where input1.mp4 has the higher resolution):

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex 
'[0:v]pad=iw:ih*2:0:0[intv];[intv][1:v]overlay=(W-w)/2:H/2[vid]'
-map [vid] -c:v libx264 -crf 22 -preset veryfast output.mp4

If you know the resolutions of the videos, you can get a nicer output. Assuming input1.mp4 is larger (or at least the same resolution), and input2.mp4 has a height of 360:

ffmpeg -i input1.mp4 -i input2.mp4 -filter_complex 
'[0:v]pad=iw:ih+360:0:0[intv];[intv][1:v]overlay=(W-w)/2:H-360[vid]'
-map [vid] -c:v libx264 -crf 22 -preset veryfast output.mp4

I think I found the answer here: https://stackoverflow.com/questions/575705/how-can-i-tile-videos-create-a-video-montage I will have a closer look into the solutions posted in that thread.