Beaver and Rosen (1978) claim that "The earliest collaborative paper found was published in 1665 and attributed to HOOKE, OLDENBURG, CASSINI, and BOYLE". But it seems that it is the earliest collaborative (= more than one author) article in their 10% sample. This interpretation is suggested by the longer excerpt of their paper: "Of the approximately 20000 or so scientific papers written in the period, a 10% random sample was taken for certain research fields. Table 2 displays the statistical results. The earliest collaborative paper found was published in 1665 and attributed to HOOKE, OLDENBURG, CASSINI, and BOYLE. Only six of the forty-seven collaborative papers produced before 1800 in the sample date from the seventeenth century".
Beaver, D., & Rosen, R. (1978). Studies in scientific collaboration: Part I. The professional origins of scientific co-authorship. Scientometrics, 1(1), 65-84.