Balance spring with two hairsprings and improved isochronism

A balance spring includes a first hairspring, a second hairspring, and an attachment member securing an outer coil of the first hairspring to one end of the second hairspring so as to form a dual balance spring in series. A curve of the first hairspring and a curve of the second hairspring each have a continuously variable pitch and are symmetrical relative to a straight line parallel to first and second planes and pass through a median plane of projection of the attachment member. Each hairspring further includes at least two counterweights so as to compensate for an unbalance formed by a mass of the attachment member and personalize an anisochronism slope of the balance spring.