Join 2 videos to play at the same time
Use

ffmpeg -i file1.avi -i file2.avi -filter_complex "[0][1]vstack" -t 10 joined.avi


This will stack the videos vertically and play the first 10 seconds of each. Remove the -t option if the videos are longer and you wish to extend the output. This command also uses the default parameters to encode the video i.e. codec will be MPEG4 Part 2 (Xvid..). If there's any audio, the track from the first video will be used and encoded as MP3.