from
Bouvier's Law Dictionary, Revised 6th Ed (1856)
SIMUL CUM, pleading. Together with. These words are used in indictments and
declarations of trespass against several persons, when some of them are
known and others are unknown.
2. In cases of riots it is usual to charge that A B, together with
others unknown, did the act complained of. 2 Chit. Cr. Law, 488; 2 Salk. R.
593.
3. When a party sued with another pleads separately, the plea is
generally entitled in the name of the person pleading, adding "sued
with___," naming the other party. When this occurred, it was, in the old
phraseology, called pleading with a simul cum.