How to play two or more videos side by side in a syncronized fashion in Linux?

Describing Florian's solutions:

If your want play 3 videos:

1) make 3 pipes

$ mkfifo cmds1 cmds2 cmds3

2) run 3 mplayers reading commands from pipes

$ mplayer -slave -input file=cmds1 -quiet file1.mp4
$ mplayer -slave -input file=cmds2 -quiet file2.mp4
$ mplayer -slave -input file=cmds3 -quiet file3.mp4

3) use tee to write to pipes simultaneously.

$ tee cmds1 cmds2 > cmds3

4) now enter commands like pause, ...

http://www.mplayerhq.hu/DOCS/tech/slave.txt

Maybe it could be done writing a simple frontend driving two MPlayer instances in slave mode, see -slave in the mplayer manpage