yes, these do tend to slide apart. i slit a couple short sections of garden hose and duct taped them to the bar next to the inner support. that keeps that one from sliding. my tower foot keeps the other one from moving.
you could also just loop a rope around the pair and tie that off to hold them together. But for the price of these things you shouldn't have to do this shade tree engineering. They do support my hull nicely though and because they flex you can tighten the boat nicely.I have owned the Yakima Evenkeel Saddles for almost two years now.
they have held everything from an Ocean Kayak Scrambler to a WS Pungo 140 to an Ocean Torque to my current to go boat; a Necky Looksha 17. The Saddles were placed on a rounded Yakima bar, they never gave me any trouble nor they came out of place. Several kayaks have traveled up 'n' down at 65 mph without incidents or dents.
even though these seem extremely simple to install it does pay greatly to read the whole instructions booklet.
I have had many others, more expensive rack attachments for my kayaks yet the Evenkeel remains as my all time / all kayak's favorite.