It takes concentration.
Do what Blizz said and kinda just cross your eyes. While concentrating on maintaining that, simply concentrate on the center image, don't really try to move your eyes, just think about it, and your eyes will focus on their own. At least this is kinda how it worked for me. More of a matter of changing your focus with your mind than physically trying it. You eyes will do it, but let them do it on their own without your input. I dunno if that even makes sense. Oh, well.