Good point, DRC.
If Michael went to trial (for, say, perjury), a judge might rule that Tom was "house counsel" and block him from representing Michael. That's what happened during Gotti's last trial when the judge ruled that Bruce Cutler was "house counsel" because wiretaps and tapes revealed Cutler counseling Gotti on Gambino Family operations that had nothing to do with Gotti's legal troubles.

But the Senate hearing was not a trial, and Michael, not Tom, was the focus of the hearings. Had the committee tried to bar Tom, he'd have raised a big stink with the Bar Association, Civil Liberties Union, etc., that would have taken the focus off Michael and cast doubt on the "fairness" of the proceeding.