Aetherna
I suppose one way to become ambidextrous would be to actually recognize the control of both the left and right hand at the same time. Though, if you're already out of the womb, it's a bit too late for that.
I don't know how one can be naturally ambidextrous, since there is no genetic code for being left/right-handed. But, supposedly in a study infants most infants who sucked their right thumb inside the womb were right-handed. The same goes for the left. Maybe if an infant were to use both, they could be ambidextrous at birth. (or something like that
sweatdrop ) Apparently, the same goes for babies that prefered to have their head tilted to the right/left when lying down.