The most significant challenge to free viewing is focal/angular displacement. When parallel viewing, the eyes are positioned for viewing at infinity. Usually, the image being viewed is placed 12 to 18 inches from the eyes. Significant and conscious effort must be exerted to allow the eyes to focus closely independently from the parallel driven urge to focus at infinity. Conversely, crossed eye viewing forces the eyes into an acute angle that would normally indicate an object at very close proximity. Consequently, considerable effort must be expended to force the eyes to focus at a distance somewhat beyond the angular cues being supplied to the brain.
Free viewing requires practice. Take your time. This is not an easy technique. Younger people tend to have the greatest success. There's something about the flexibility of youth. I learned free viewing when I was in my mid teens. I drive my optometrist nuts! I can converge and focus on impossibly misaligned images that are used to diagnose serious vision deficits. Then I have to take several minutes to explain the techniques that I have practiced and developed to a very high degree. This technique cannot damage your eyes. When your mom told you to 'stop looking cross-eyed or your eyes will stick like that' she was merely trying to ensure you didn't look like a moron when you went to visit grandma. Sure, if practiced to excess initially you'll get a splitting headache. Take it easy. Try it for a few minutes, take a break and come back to it an hour or two later. It can take months to develop comfortable and flexible control of your viewing mechanism. I believe it's worth the effort, and certainly it's at least worth a try.