Wow, this is interesting. I'm a righty. I'll eat munchies left handed or right, and when I cut my food I hold the fork in my right hand and cut with my left. It's easier that way (cut, eat, cut, eat -seriously why switch?) Someone once told me that's how then "detect spies" or some bs like that. It sounded like whoha then and sounds like whoha now.
My Great Grandmother was a lefty so I've watched my three year old since she was old enough to hold things. She eats with either but ALWAYS holds the pencils and crayons in her right hand.
I once knew someone who was both. He was a righty to start with but when he was little he wanted to be just like his brother so would mime him - mirror image style when they would color, etc. As an adult he was a lefty all the time.