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.