Two or more insurers authorized to do in this State the business of fire insurance may, with the approval of the Commissioner of Banking and Insurance, issue a combination form of fire insurance policy which shall contain the following provisions:
(a) A provision substantially to the effect that the insurers executing such policy shall be severally liable for the full amount of any loss or damage according to the terms of the policy, or for specified percentages or amounts thereof, aggregating the full amount of such insurance under such policy.
(b) A provision substantially to the effect that service of process, or of 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.