What's the point for the same company to have two websites doing the same thing?
1.Streamlined efficiencies, synergies, and economy of scale.
2. They don't do the same thing, Kayak doesn't actually sell tickets (AFAIK they just refer to sites that sell), Priceline probably hopes they can compel people shopping on Kayak to ticket through them (vs. through the airlines site directly or from one of their competitors).