What is the test for a plaintiff to bring a cause of action against defendants' attorneys?

It is difficult to know where to begin, given the confusing and probably mistaken "Plaintiff and Plaintiff's Attorney" language in the first sentence, but we will take the leap of presuming this cause of action was intended to be against defendants' attorneys. An attorney's duty to his or her clients is one of "undivided loyalty," (Mason v. Levy & Van Bourg (1978) 77 Cal.App.3d 60, 66), but that duty does not extend to third parties or opposing parties. To the extent he argues defendants' attorneys gave them bad advice, he has no standing to assert such a claim. As such, his "professional negligence" claim must fail.