49 CFR 1103.16: Adverse influences and conflicting interests.

(a) At the time of the retainer, the practitioner shall disclose to the client all circumstances of his relations to the parties, and any interest in or connection with the case.

(b) It is unethical for a practitioner to represent conflicting interests, except by express consent of all concerned given after a full disclosure of the facts. Within the meaning of this section, a practitioner represents conflicting interest, when on behalf of one client, it is his duty to contend for that which duty to another client requires him to oppose.

(c) The obligation to represent the client with undivided fidelity and not to divulge secrets or confidences forbids also the subsequent acceptance of retainers or employment from others in matters adversely affecting any interest of the client with respect to which confidence has been reposed.