from
Bouvier's Law Dictionary, Revised 6th Ed (1856)
INSIMUL COMPUTASSENT, practice, actions. They accounted together.
2. When an account has been stated, and a balance ascertained between
the parties, they are said to have computed together, and the amount due may
be recovered in an action of assumpsit, which could not have been done, if
the defendant had been the mere bailiff or partner of the plaintiff, and
there had been no settlement made; for in that case, the remedy would be an
action of account render, or a bill in chancery. It is usual in actions of
assumpsit, to add a count commonly called insimul computassent, or an
account stated. (q. v.) Lawes on Pl. in Ass. 488.