Does the multiplicity principle apply to an action against a lawyer?

Much of the evidence involved in the plaintiffs’ present action against the defendants would be relevant to an action by them against the lawyer. Recognition of the multiplicity principle would therefore suggest that, if the plaintiffs have a valid cause of action against the lawyer, they should be allowed to prosecute it in tandem with their claims against the existing defendants. That is a valid argument but the German v. Major principle referred to above must take precedence.