Can an insurer sue at law in cases where the extent of the loss is not covered by the insurance policy?

In Offer v. Superior Court, 194 Cal. 114, 118--119, 228 P. 11, 13, it is stated: 'It may be suggested that to permit the insurer to sue at law in cases where the extent of the loss is not covered by the insurance policy would result in splitting a cause of action sounding in tort, and thus subject the defendant to two suits upon a single cause of action. It seems, however, that this result may be avoided by joining the insured as a coplaintiff * * * or as a codefendant * * * and if the plaintiff fails to do so the defendant may protect himself by pleading such defect of parties, either by demurrer or by plea in abatement. * * * It would seem to follow that, if the wrongdoer in an action against him by the insurer for the amount of damages covered by the insurance does not demur or enter a plea in abatement because of nonjoinder, the defect is waived, and he might be held liable to the insured in another action for the amount of the loss not covered by the insurance policy.'