The Supreme Court of Pennsylvania has suggested by way of dicta that it can be unethical for an attorney to act as trial counsel in a case wherein his partner will appear as a material witness.
Weiherer v. Werley, 422 Pa. 18, 221 A.2d 133 (1966).
John R. Kenrick,
Legal Ethics - Canon 19,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol5/iss3/13