Is diverging harder? I find it easier. Maybe it is from long ago practice on stereograms, but I'm curious if it could be due to neurological/physiological differences.
Crossing is easier because you can simply hold your finger in front of your eyes and look at that for practice.
Diverging requires you to look past the image, meaning you have nothing to really look at, which makes it difficult to figure out what your eyes are even supposed to do.
Those stereograms aren't helping much either, since they look like nothing until you get it right. With cross-eye you have instant double-vision that you just need to align.
Cross-eye also works across much larger distances, diverging fails when the images are too far apart.
It depends on the image. If the two images are too far apart then it could require your eyes to diverge, and not to just converge slightly less. That might be impossible.
Diverging is way way easier for me, but I am positive that's because of the 10's of hours (at least) that I spent staring at magic eye images as a child.
