change aspect ratio without stretching video ffmpeg
Have you tried uploading it to Youtube? Their player window will be a fixed 16:9 ratio (on YT.com), but they should accept source videos of any ratio, and simply center it in their window during playback.

Anyway, here's the method:
ffmpeg -i "a.mov" -i "b.mp4" -filter_complex "[0:v]scale=iw*sar:ih,pad=if(gt(dar,16/9),iw,2*trunc(ih*16/9/2)):if(gt(dar,16/9),2*trunc(iw*9/16/2),ih):(ow-iw)/2:(oh-ih)/2,setsar=1[a];  [1:v]scale=iw*sar:ih,pad=if(gt(dar,16/9),iw,2*trunc(ih*16/9/2)):if(gt(dar,16/9),2*trunc(iw*9/16/2),ih):(ow-iw)/2:(oh-ih)/2,setsar=1[b];
[a][0:a] [b][1:a] concat=n=2:v=1:a=1 [v] [a]" -map "[v]" -map "[a]" -preset veryfast -crf 18 "merged.mp4