I have a big problem crossing my eyes too while having no problem with the parallel view way seeing stereograms. I am actually going to stop trying as my eyes started to hurt.
For me, what's difficult is holding my right eye closed without my left eye drifting to look at my nose. My right eye's good, I can move it and focus on anything within my (now peripheral-limited) view... but the left is wonky. I think I learned how to wink (and hold it) with the right really early, by age 3 or 4, but the other side I never tried until I was pre-teen... some sort of muscle atrophy?
You can also tell if your head's level, just by crossing your eyes. If the two images are diagonal to each other, then your eyes/head aren't level. I have no idea what the possible use for that would be.
Which one makes things become bigger? I learned that one first and then later figured out the one that makes the mixed image smaller (cross eyed I think?). Now I cannot do the big one anymore.
