Well, first remember it's not crossing your eyes, which is convergence. This is divergence, and only a little is necessary (same amount used for Magic Eye pictures).
Second, well, you don't HAVE to be able to, but it helps a lot. Some people can't get the blip page, and use the soft focus "X" method instead, but in my opinion it's just not as good. It's the only possible one, however, for people who are blind in one eye. I hate saying you can get away without it, since it may encourage people to settle for the "X" method, but no, you don't need it if you absolutely can't get it.
It's really not hard to get, though...have you tried the floating sausage? Did that work?
-Ramon
