SO LATE as 1822 Sir John Nicholl is reported to have said in Hobson v. Blackburn, that a mutual, or conjoint will is an instrument "unknown to the testamentary law of this country; or, in other words, that it is upknown, as a will, to the law of this country at all. It may, for aught that I know, be valid as a compact." In Darlington v. "Pulteney, Lord MANSFIELD said, "there cannot be a joint will." Following these distinguished and learned judges, Jarman and Williams in their classical treatises accepted the statement of Sir John, and some early American cases refused probate to so-called "joint wills."
Goddard, Edwin C. (1900-1935). "Mutual Wills." Mich. L. Rev. 17 (1919): 677-87.