Combining / merging / mixing multiple audio files using MP4Parser on Android

Question

I have two MP4 audio files and one MP4 video file. I need to combine them to play video and two sounds at the same time (Parallel, Not concatenate) to get only an MP4 file.

For example, I have an MP4 audio file with a voice, another with music, and another with video. I need them to play together like a video.

I am trying to add multiple audio tracks to a movie with MP4Parser with:

video.addTrack(audioTrack1);
video.addTrack(audioTrack2);

But when I play, I hear only one of them. Maybe MP4Parser is working correctly, and I did not understand this functionality, so I need an answer.

Is it possible to do this with MP4Parser, or am I mistaken and this is not possible?

If possible, please could you give me an example of working code?

Thanks in advance.