Throw an enemy, then hit it with another enemy while it's still upside-down to make both enemies shoot straight up. One way to do it is to stand facing a wall or pillar, holding an enemy while standing on top of another enemy. Press B twice quickly, and if you bounce the first one off the wall you'll have a good chance of pulling off this bug.

Another method is to throw a Shyguy into quicksand, then quickly throw another Shyguy onto the one drowning in the quicksand (try this in World 2-1).