Your current 2003 Ford Explorer factory roof rack crossbars are not rated to safely handle the weight of kayaks. Instead you should mount the Thule 450 CrossRoad roof rack system to the Ford Explorer roof side rails (the pair of bars that run parallel to each other from the front to back of the roof). The 450 CrossRoad converts your factory rack into a sturdy base rack system capable of safely transporting multiple kayaks. Includes four 450 Crossroad Feet that attach directly to the side rails and also a pair of Thule load bars that connect to the 450 CrossRoad Feet. Choosing a longer length pair of bars such as the 58 inch bars will also provide enough crossbar space to carry a couple of kayaks.
http://www.orsracksdirect.com/thule-450-complete-car-roof-rack.html
The Thule 835PRO Hull-A-Port is a great universal kayak rack attachment that uses a j-cradle design for easy loading/unloading of your kayak and attaches to the Thule 450 CrossRoad load bars. Includes all hardware, 2 straps, buckle bumpers and a pair of Thule 855 Quickdraw ratcheting tie down straps for securing boat bow and stern. The all-new design of this carrier also allows the cradle to fold down when not in use, avoiding any clearance issues such as a garage etc. Get two for carrying two kayaks.
http://www.orsracksdirect.com/thule-835pro-hull-a-port-kayak-rack.html
The Thule 835XTR Hull-A-Port is exactly the same as the 835PRO except the cradles cannot be folded down.
http://www.orsracksdirect.com/thule-835xtr-hullaport-kayak-rack.html
The Thule 883 Glide and Set is another excellent option and offers an easy way to load your kayaks mated to the best fitting saddle carrier. This system is a complete set for carrying a kayak. Includes one pair of 875XT Hydro-Glide saddles (for the rear load bar) and one pair of 878XT Set to Go saddles (for the front load bar). Includes all hardware, 2 straps, buckle bumpers and a pair of Thule 855 Quickdraw ratcheting tie down straps for securing boat bow and stern. Get two for carrying two kayaks.
Thule 883 Glide & Set kayak rack