In case any employee for whose injury or death compensation is payable under this article shall, at the time of the injury, be employed and paid jointly by two or more employers subject to this chapter, such employers shall contribute the payment of such compensation in the proportion of their several earnings liability to such employee. If one or more, but not all of such employers, should be subject to this article, and otherwise subject to liability for compensation hereunder, then the liability of such of them as are so subject shall be to pay the proportion of the entire compensation which their proportionate earnings liability bears to the entire earnings of the employee. Nothing in this section shall prevent any arrangement between such employers for a different distribution, as between themselves, of the ultimate burden of such compensation.