How to merge two .mp4 videos of different resolutions

I need to embed videos on youtube via the API, and to make the process a little more practical I want to merge my vignette with the videos before posting on youtube. However, the videos have different resolutions and different characteristics, so the solution I found leaves the final video with audio or streaming problems.

I ended up finding the following solution, which works well, but is extremely slow, see:

$code1 = 'ffmpeg -i vinheta.mp4 -qscale 0 vinheta.mpg';
$code2 = 'ffmpeg -i movie2.mp4 -qscale 0 movie2.mpg';
$code3 = 'cat vinheta.mpg movie2.mpg | ffmpeg -f mpeg -i - -qscale 0 -vcodec mpeg4';

system($code1);
system($code2);
system($code3);

This solution works well and without breaks, but it really takes a long time, some video takes up to 2 hours to convert, and after that the video is extremely heavy

I've researched it in several places, tested all kinds of code and this was the only one I found that works without leaving the final video with audio or video problems regardless of the resolution.

Another interesting solution is as follows:

$code1 = "ffmpeg -i vinheta.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts temp1.ts";
$code2 = "ffmpeg -i 4.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts temp2.ts";
// now join
$code3 = "ffmpeg -i \"concat:temp1.ts|temp2.ts\" -c copy -bsf:a aac_adtstoasc output.mp4";

system($code1);
system($code2);
system($code3);

The output file works perfectly fine. But only on my computer. When I send it to youtube, the audio of the second video keeps chopping

Is there any simpler and lighter way to do this without breaking my final video?

Solution

regarding the second code that was chopping the audio on youtube, I managed to solve it just by changing the codec. So now it's working perfectly fine as follows:

$code1 = "ffmpeg -i vinheta.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts temp1.ts";
$code2 = "ffmpeg -i 4.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts temp2.ts";
// now join
$code3 = "ffmpeg -i \"concat:temp1.ts|temp2.ts\" -c copy -acodec aac output.mp4";

system($code1);
system($code2);
system($code3);