Concatenate two mp4 videos into one file

There are two ways you can do this.

1. Using Mencoder

mencoder file1.mp4 file2.mp4 -ovc copy -oac copy -of lavf format=mp4 -o output.mp4

2. MP4Box

MP4Box -add file1.mp4 -cat file2.mp4 output.mp4 

Note the use of '-add' for the first file and '-cat' for second file; if you mistakenly use '-add' for the second file - you will lose the first file's content.