What size kayaks? If the two kayaks together are narrower than your roof rack, then the cheapest way to carry two kayaks is to set them side-by-side on the roof rack.
That's probably unlikely if you have the stock RAV4 roof rack bars, so the next simplest option would be to get one set of the "J" style cradles (like
the Salamander J-Stroke) for the first boat, and load the second boat flat on the bars. Another alternative is to use the "stacker" style rack (like
the Malone Stax) that holds both boats on their sides. Other companies like Thule (Hull-a-Port models) and Yakima (Hullraiser models) make very similar racks at slightly higher prices, but are easier to find gently-used on Craigslist. The kind that fold down out of the way are nice if you need to fit in a garage.
You can easily spend $500 or more on kayak rack accessories. Paying more will influence how much effort is required to load/unload the boats, as well as the level of protection for the boat against damage. Unless you have a really nice kevlar/carbon kayak, or transport the kayaks every day, I'd recommend sticking with the factory roof rack bars and an affordable "J" or "stacker" setup.