The factory cross bars may not be rated to carry the weight of two kayaks if they are heavier, say plastic and longer length. You need to check that.
That said, Jack speaks above of the more extensive options you have by investing in a third party system of towers and cross bars. Any of the major manufacturers he lists will use cross bars that likely have a higher bearing weight limit than your factory cross bars, and you can get more length, so the whole situation is better secured.
All of the listed manufacturers are good and have fit information on their web sites, to tell you what components you need by make and model of car. The one thing you may want to consider as to specifics is that J-racks can be more challenging to slide a boat into on a taller roof than something that can be approached from wider angles, like stackers or saddles.
Deck Rigging Gear