When two or more authorized companies unite in the issue of a joint policy, the heading thereof may show the severalty of the contract, and the policy shall show the proportion of premiums to be paid to each, and the proportion of liability which each assumes, and shall contain a provision to the effect that service of process, or any notice or proof of loss required by such policy, upon any of the insurers executing such policy, shall be deemed to be service upon all such insurers.