The 12-letter word meaning “having the ability of using both hands with equal skill or ease” is ambidextrous.
The word comes from Latin roots: ambi- means “both” or “on both sides,” while dexter means “right” or “right-handed.” Because the right hand was traditionally associated with skill and dexterity, ambidextrous developed the sense of being equally capable with either hand. The noun form is ambidexterity, and a person with this ability may be called ambidextrous.
Ambidextrous does not mean that someone merely switches hands occasionally. In its strict sense, it describes unusually balanced skill in both hands, although people vary considerably in how evenly developed that ability is. It is also different from cross-dominance, in which a person prefers different hands for different tasks—for example, writing with the right hand but throwing with the left. A common spelling mistake is leaving out the second “d” in the middle: the correct spelling is a-m-b-i-d-e-x-t-r-o-u-s.