Does an Attorney have an obligation to disclosure "Other" Clients that may have a Conflict of Interest?

An attorney has an obligation not to represent clients with conflicting interests unless both clients waive the conflict, and even then there are some conflicts that cannot be waived.  However, if the attorney does not accept the new representation due to a possible conflict, or does not think that there is a conflict, I am aware of no requirement that he disclose to one client the identity of another.  To the contrary, the first client would probably not want its identity disclosed, and the attorney would be bound to respect the client's wishes to the extent possible and legal.