append help

[mny459]

I use MediaRecoder to recode video, first, I use back camera to recode video1(MP4), and then use front camera to recode video2(Mp4). Finnally, I append video2 to video1 as video3, in video3, video1's part seems ok, but it up down in video2'part. I do not know how to fix it. Could you give some advice? BTW, append method is exact as example

[ZhuoJunErGe]

Adjust the size of the video output consistent on it