Wiktionary
mutual will
n. (context legal English) A pair of identical documents executed by a married couple, one signed by the husband, the other by the wife, intended to create an irrevocable contract insuring identical dispositions of their property upon the death of either or both.
Usage examples of "mutual will".
In hardly more than a moment they had their mutual will of him, top and bottom.
They had learned to work together in this fashion, Gilafas the hands behind their mutual will.