- Professionals use timecode-synced equipment for this job. Low budget productions do this with simple audio marks at the beginning and the end of a take. You can do this simply by clapping with your hands. But using a slate gives you a more significant mark (peak).

 - The main audiotrack is a separate mix. This is the reference. The videos will be synced track-by-track to this reference. Usually this will be done manually. But **Final Cut Pro X** can also do this automatically with great success. 

 - The splitscreen itself is nothing special. Just adjust size, crop and position of a track or clip as you like.