I've always been able to do this too even with a pretty large distance .. it's the same technique i use for those magic eyes. I think I do it backwards from what they always recommend -- I go crosseyed instead of walleyed. Technically I think this is supposed to cause the depth perception to invert (so things that are supposed to appear to be coming towards you are going away from you), but I read somewhere that the brain just corrects for that (which is pretty slick).