Extract Audio/Video - Join Again

[omidsolo]

Hi,
I have 2 .mp4 files using Advanced Video Codec.
I want to extract their Audio/Video in 2 separate files, without any kind of encoding to prevent quality loss.
Now that I have separated Audio/Video files, I wanna join one's Video to the other one's Audio.
This join also must NOT have any kind of encoding to preserve the original quality.
I don't know which is the best free tool for doing the 1st and 2nd task?
Please advise,
Thanks

[sneaker]

You can probably use MKVToolNix GUI or ffmpeg. Drop both mp4 files into MKVToolNix GUI, disable the tracks you don't want to keep and click "Start multiplexing". If the video/audio don't match you can also add a delay to a track or change the video fps.

[omidsolo]

MKVToolNix was able to perform both operation.
So both in extracting Audio/Video parts and re-join them MKVToolNix just copy and does not do any encoding yes?
Thanks.

[sneaker]

Correct. But for MKVToolNix it is a single operation, you do not need to extract first.