There were also bi-pack magazines for Mitchell or Acme cameras, which is what Kuroda was probably talking about, looks like the camera has four reels attached to it.
http://www.zen171398...ck in smoke.jpg
As Mark says, an optical printer is basically a camera pointed at a projector, so a bi-backed Mitchell is basically the camera side of an optical printer but pointed at other things like miniatures, matte paintings, etc. Not better or worse, just a different workflow.
You could, for example, shoot a spaceship model against black, keep that take undeveloped, shoot a second identical take of the spaceship silhouetted against white, develop that take and make hi-con b&w positive version for a hold-out matte, then load the hold-out matte into the bi-packed camera with the undeveloped take of the model against black behind the hold-out matte (black where the spaceship model is, clear all around) and then point the camera at whatever you want as the background, which will only get exposed around the spaceship model but be matted out of where the spaceship is in the frame. If you have a motion control set-up, you can do a camera move on the spaceship model and repeat it to shoot the hold-out matte.
Or you could use a bi-pack camera to shoot matte paintings like Douglas Trumbull did. You shoot the live-action and let's say you want to put a matte painting of the city at the top of the frame later. You make a work print of the developed live-action plate and project that onto a glass with white behind it and then draw an outline of where the painting will go into the frame. Then you paint the matte painting, with clear areas for the live action on the glass. You then make an interpositive of the negative of the live action and load that into the bi-pack camera as the front piece of film with intermediate dupe stock loaded behind it. You then shoot the matte painting with no light on it but the clear area behind it lit up with an even white field so that the white area exposes the image on the first element onto the second element, but where the matte painting is silhouette, that area on the new internegative is unexposed. Then you take out the front piece of film in the bi-pack camera and shoot the matte painting, this time lit up with the live action area in black on the painting (by putting black velvet behind the glass where before you had a white field). Now you can expose this painting onto the internegative and it will appear in the unexposed portions of the live action element. (I'd have to dig up my Blade Runner issue of Cinefex to see if I described that correctly).
The only trouble with this technique was that it required using intermediate stock in a camera, which is very low in ASA and is designed to see the colors of a piece of film, not reality, so Matt Yuricich had to paint his paintings in weird off-colors that would look correct to the intermediate stock, not for camera negative stock, and then use a ton of light on the painting to expose it. This kept him from using a technique he liked on "Logan's Run" of painting over large color photographs, because the paper prints could not take the heat from this lamps used to expose onto intermediate dupe stock.