The prior written authority, policy of insurance or copy of evidence that insurance has been bound referred to in Section 1764.2, shall contain authentication by all persons assuming any risk of loss, and, if there is more than one such person, both it and any document issued or certified by the placing broker pursuant to subdivision (b) of Section 1764.3 , shall contain a specification of whether their obligation is joint or several, and if the latter, the proportion of the obligation assumed by each person.