Ever since I was a child addicted to the "magic eye" stereogram books, I've always diverged (not crossed) my eyes for spot-the-difference puzzles.
Also, if you're doing it on a piece of paper, hold a pen in each hand spaced right so you see the middle (3rd) hand in the middle combined image, and move both hands in sync to circle all the differences. Kind of a cool way to point them out to someone else.
The difficult puzzle took me about 10 seconds here since I was looking for more than one difference. I saw the first difference in about 1 second.