As I understand it - any immortal can supermax more than one skill, but each immortal has to have their own personal private skill that nobody else has supermaxed.
In TallStar's scenario, if fishing was the skill "claimed" by Sim A, then Sim A could supermax gardening, too, as long as no other immortal claimed it as their unique skill. If, when Sim A supermaxes gardening, Sim B has already used gardening for their unique skill, the dynasty would fail.