Double click one of the video files that you would like to join to open it in QuickTime.
Move the scrubber to the position in the clip where the other video will be inserted.
Drag another clip into the QuickTime window to insert it before the scrubber.
Select Save As from the File menu
Enter a name for your the combined video file then click the Save button.
QuickTime will now save the joined video.
After a couple minutes the avi video files will be joined and ready for playback.