• 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.

In general you should be able to do that with any video editing software.

• 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.