Can an attorney continue to represent a client whose interests are adverse as to unrelated matters?

"The principle of loyalty is for the client's benefit; most courts thus permit an attorney to continue the simultaneous representation of clients whose interests are adverse as to unrelated matters provided full disclosure is made and both agree in writing to waive the conflict. [Citation.] But this class of cases is a rare circumstance, typically involving corporate clients, and overcoming the presumption of 'prima facie impropriety' is not easily accomplished." (Flatt v. Superior Court, supra, 9 Cal.4th at p. 285, fn. 4; emphasis in original.)