Having worked in the media buying world for TV I can tell you this is very deliberate and the sharing of costs is carefully planned. Often, the vendor will act as mediator and help bridge the responsibility that all parties (agencies + brands) will bring to the table.

There is definitely a financial advantage to this as you get 2 brands in the cost of one spot BUT the downfall is that the process is much harder because you’re doubling the cooks in the kitchen.