How can I play several files at the same time with QuickTime X?
What does "play several files at the same time" mean to you?
If you are referring to the playback of multiple files within a single player, then use QT 7 Pro to place all data in a single file and offset the various display areas in such a way that they they do not block each other and then play the file normally in the QT X player app.
If you are referring to the simultaneous playback of multiple files using the QT X Player app, then open the various files and move the players to screen positions so they can all be seen at once before pressing the Command-Return shortcut key combination to start the simultaneous playback of all players at once.