Visited a soviet silo just outside of Pervomais'k, Ukraine (thoroughly recommend a visit if you find yourself in that neck of the woods, you can sit atop a Satan missile and wave your cowboy hat until a burly Ukrainian tells you to get off before he shoots you) - it's a two-key system, with two buttons, on opposite sides of, well, a really tiny room. While one could reach both buttons and keys simultaneously, the idea was that the keys would be carried by separate people - however in practice, they both hung from a peg on the back of the chair, meaning that someone could have theoretically initiated a launch singlehandedly. Theoretically, because the other fun fact revealed was that most of the time they were nothing like launch ready, and perennially struggled with equipment failure.
Oh, and don't forget that the super-secret launch code for minuteman missiles across the US was... uh, 00000000, for several decades.