Apparently it was in the manual, but I'm just learning it now.

Found this out completely by accident once after my sister and I played some Mario.

I had the 2nd controller still plugged in, and while shooting the ducks I stepped on the controller and the ducks moved differently.

From the on, every time someone wanted to play duck hunt I would grab a second controller and make it harder for them.

Bonus knowledge: the original game works by a light-sensitive sensor in the blaster tip, and when you pull the trigger, the screen goes black and a white square appears whee the ducks were, in a specific order. If the game controller detects the light square, it counts as a "hit" on whatever duck was in frame. You can cheat by pointing the blaster at a white light and pulling the trigger. It will just go through them one by one as you squeeze, thinking the light is the duck square.