Two or more companies authorized to transact business in this State may issue a combination or group form of policy, using a distinctive title therefor, which title shall appear at the head of such policy followed by the titles of the companies obligated thereupon, and which policy shall be executed by the officers of each such companies; provided, that before such companies shall issue such combination or group policy, the title of such proposed policy and the terms of the additional provisions thereof, hereby authorized, shall have been filed with the Director, which terms, in addition to the provisions of the standard policy and not inconsistent therewith, shall provide substantially under a separate title therein, as follows:
     (a) that each company executing such policy shall be liable for the full amount of any loss or damage, according to the terms of the policy, or a specific percentage thereof;
     (b) that service of process, or of any notice or proof of loss required by the said policy, upon any of the companies executing the same shall be deemed to be service upon all; and provided further that the unearned premium liability on each policy so issued shall be maintained by each of such companies on the basis of the liability of each to the insured thereunder.