Most wills are executed by one person and may be amended, revised or revoked at any time prior to the testator's death. Occasionally, use is made of wills that are "joint" in that the instrument is signed by two or more individuals (usually spouses) and contains in one document the testamentary bequests of each testator. Such wills are, in effect, the separate wills of each of the testators and can be revised or revoked by any of the testators as to that person's property.
Wills that are both "joint" and "mutual" contain a contractual obligation that binds the parties to dispose of their properties that are subject to the contractual obligation according to the terms of the will. The contractual feature becomes binding at the death of the first testator to die; the surviving testator's interest is effectively reduced to a life estate with the remainder interest passing to the beneficiaries named in the will.
"Joint and Mutual Wills: A Mischievous Concept,"
Agricultural Law Digest: Vol. 5
, Article 1.
Available at: http://lib.dr.iastate.edu/aglawdigest/vol5/iss19/1