Under the laws of wills and testamentary dispositions, joint wills cannot be revoked after the demise of one of the joint testators. Extending the principle in this case would suggest that nominations made jointly cannot be revoked after the demise of one of the joint holders.
However under section 109A the nomination made are to take effect notwithstanding testamentary dispositions, or otherwise.
Therefore on demise of one of the joint holders the remaining joint holder would become sole member. In such case, fresh nomination in prescribed manner could be made revoking the old nomination.