I agree that the best bet is to call your local building department. Local amendments to the code or local policies may be enforced.
That being said, the International Residential Code has no requirement for a center handrail on a stair of any width. Center handrails are never mentioned. A stair that requires a handrail requires only one, and it could be on one end of a 20' wide stair and completely satisfy the minimum code requirement.
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