As mentioned, 645 would be half frame for 6x6. It is normally a vertical format. I think I only heard of one in a horizontal format with the film traveling vertically. Many folding cameras had inserts to transform them from 6x6 to 645. They had sometimes double windows on the back of the camera, until Kodak began putting markings on the paper for 645. It is rare to find those cameras which accepted the mask on sale with the mask. I have a Welta an a Fujica with the mask. But I am not a fan of 645 so I don't really use them that way.
As to double exposure prevention, the closest to that I can think of would be those that have auto-wind, that is they advance the film one frame and then stop. But really is for doing that advance without needing to use the window on the camera back. I don't know how many like that were made. I happen to have two, a Welti Perfekta folding TLR, and the Mamiya Six. But again, I don't know of any that self cocked the shutter.