There are lots of solutions, but this depends on what kind of vehicle you have -- sub compact, SUV, truck, etc. This affects vehicle-rack weight limitations and methods to mount the kayaks (side our rear), which in turn bears on which equipment you would want.
The simplest and least expensive way to carry 2 overhead is with simple 66" or 72" wide load bars with the boats side by side, upside down. The question is whether that is feasible for you -- is this too wide for your car, are you mounting gear on the kayak rails that prohibits this arrangement, can you lift the boats overhead from the side or would a rear mount be more favorable?
So give us a few more details about the vehicle and how you envision loading the boats.
