Don't take random youtube videos as fact. That part of that video was incorrect. The unmanned SpaceX Dragon has three pairs of computers used for flight control (so six flight control computers), and has 54 processors in all. The Falcon 9 launch vehicle has another 30+ processors. See http://aviationweek.com/blog/dragons-radiation-tolerant-design. Orbital's Cygnus also uses more than two computers for flight control.
The Shuttle had four primary flight control computers, plus one more as a completely independent backup flight system.
Visiting vehicles to the Space Station, whether crewed or uncrewed, must protect against any two failures creating a catastrophic hazard on the Space Station. You aren't going to get two fault tolerance with only two components.
Crewed vehicles add the additional requirement of being two fail safe (no one on the vehicle dies even if two failures happen).