Avidemux

avidemux2_cli --load a.mp4 --append b.mp4 --output-format mp4 --save output.mp4  

Yes Yes

Links

append with command line
avidemux.org/admWiki/doku.php?id=using:command_line_usage
superuser.com/questions/76228/how-do-i-merge-mp4-files-without-audio-going-out-of-sync