I use this when I need to transport three or more yaks
http://www.autoanything.com/roof-racks/69A5913A0A0.aspx
It works great
However when I only transport 2 they fit side by side on the rack. You may be supprised to find that they fit.
I use The Thule Xsporter though on a F150, I'm not sure on the Yakima