In Johnson ex rel. Johnson v. JF Enterprises, LLC, 400 S.W.3d 763 (Mo. 2013), the
court examines whether the order in which documents are signed matters for parol evidence rule/merger clause purposes, when they are all signed at the same sitting. The court expresses the correct answer:
“The parties are incorrect in suggesting that the order in which the documents were placed by the parties in the pile of documents affects the enforceability of the merger clause or the arbitration agreement.”
Trying to find a difference depending on which of the two documents was signed first at a single sitting would be hyper-technical.
The opinion goes on to discuss construction of multiple agreements, proper analysis of which involves a number of complexities not fully examined in Johnson; some of those matters are the subject of commentary at Royce de R. Barondes, Side Letters, Incorporation by Reference and Construction of Contractual Relationships Memorialized in Multiple Writings, 64 Baylor Law Review 651-720 (2012). As discussed there, some New York authority appears to stand for the appropriate proposition that an express contemporaneous manifestation that the instruments are to be separate documents should negate application of the principle that simultaneous writings should be construed as one. By comparison, Missouri authority of a few years ago, Wilson Manufacturing Co. v. Fusco, 258 S.W.3d 841 (Mo. Ct. App. 2008), is somewhat strained in attempting to examine an understanding memorialized in multiple writings.
Update 8/31 for typo.