To clarify:
You are only ambidexterous if you can use both your hands, equally good, at the same time. There are no such thing as an ambidexterous person with a "dominant" hand.
Double-"handness" on the other hand, is usually an effect of a known learning-disability (if you were born with the ability). Of course, with some practice, you can learn how to use both of your hands equally good.
I am right-handed, and I can use my left hand for a whole bunch of tasks as writing, but I am not double-handed or ambidexterious, *because* I can use my right hand better than my left hand.
I doubt anyone here is trully ambidexterious.