Every insurance company shall conduct its business in this state in its own name, and the policies and contracts of insurance issued by it shall be headed or entitled by such name. Two or more companies may jointly issue an underwriter’s policy, upon which must appear the names of the companies guaranteeing the same, and such companies shall be jointly and severally liable thereon; Provided, this limitation shall not apply to any insurance company admitted to this state and issuing an underwriter’s policy prior to the passage and approval of this chapter, nor, in the discretion of the Department of Insurance, to any insurance company desiring to issue an underwriter’s policy after the passage and approval of this chapter.