Having similar acuity in both eyes made a huge difference for me. I'm somewhat astigmatic but was able to do this eye "uncrossing" trick just fine. I had a sign

Having similar acuity in both eyes made a huge difference for me. I'm somewhat astigmatic but was able to do this eye "uncrossing" trick just fine. I had a significant loss of acuity in one eye and that's what left me unable to do this (or to watch 3D movies).