I ran into this doing 1:1 projection prints. When distances A and B above are nearly equal, you can focus through both focal points. It is pretty cool because as you focus, the image becomes clear, then as you continue to move the lens stage, it gets blurry and then comes into clear focus again as you pass from slight enlargement to slight reduction. Once in the territory of 'slight reduction,' raising the head makes the image smaller.
I think my first post answered the question although possibly in different terms.
Regarding making 1:1 prints in an enlarger: I sometimes would get orders for several hunded prints from one negative. Using an enlarger was at least ten times faster than contacts with fewer artifacts to spot. With the Wild enlarger I used, it was very difficult to tell the difference in quality.
Why are there no speaker jacks on a stereo camera?
just out of curiosity (and ignorance), why would you make 1:1 projection prints if you could simply make a contact print? no glass easel available? easier dodging and burning?
I was actually comparing the two methods.
Since projection prints are much easier to make in my darkroom, I wanted to compare some 1:1s to to contact prints. I'm not going to comment on which looks better at this time. I know my Rodenstock 300mm lens field flatness deteriorates at 1:1, thus requiring f16, so the tests have not been 'fair.' A process lens would be better, but I don't have one.