Comment by neRok

Joining videos together sounds easy, but there's tons of ways it can go wrong! You've got time bases to consider, start offsets, frame/overscan crops, fps differences (constant vs variable), etc. And even though your videos might both be h264, one might be encoded with B frames and open GOP, and the other not, and that might cause playback issues in certain circumstances. Similarly, both could be AAC audio, but one is 48kHz sample rate, the other 44.1kHz.

Someone else mentioned Lossless-Cut program, which is pretty good. It has a merge feature that has a compatibility checker ability that can detect a few issues. But I find transcoding the separate videos to MPEG-TS before joining them can get around many problems. If you fire up a RAM-Disk, it's a fast task.

  ffmpeg -i video1.mp4 -c copy -start_at_zero -fflags +genpts R:\video1.ts;
  ffmpeg -i video2.mp4 -c copy -start_at_zero -fflags +genpts R:\video2.ts;
  ffmpeg -i "concat:R:\video1.ts|R:\video2.ts" -c copy -movflags +faststart R:\merged.mp4