6 of this article or under the terms of the dispositive instrument, is created in two or more donees, all must unite in its exercise, unless the instrument creating such power provides otherwise. But, if before its execution, one or more of such donees dies or becomes incompetent, such power may be exercised by the survivor or the competent donee, unless such exercise is explicitly barred by the terms of the instrument creating such power.
New York Legal Code