Based on what I see in the photos at the link, I suspect this was done with a combination of multiple exposure and a partial-frame mask in front of the lens, dividing the frame into four roughly equal parts.
You'd mount the mask with the opening in position one, expose, cock shutter, move mask opening to position two, expose, etc. until you've exposed all the positions your mask leaves open once each, then advance film. This is akin to the "black card" technique for making someone into twins that was published by Kodak as long ago as the 1940s (and used in film-making before 1925). Because the mask is out of focus, the edge between different exposures will be very soft, and if the mask edge is perfectly aligned from one exposure to the next, there won't be any actual overlap. If it's not, there will be.
