In my business I teach both sea kayaking and SUP. I'm a mobile business so I meet clients at the beach and have to sometimes carry both at the same time. I recently came up with a rack idea to work with both craft.
In the pics below I'm using foam pipe insulation over my Thule square rack bars. Cut into sections, each section is secured to the bar with bungees. In the middle section, I left a one foot wide foam tube section to be removed using velcro strips. In the last two photos, you can see where I removed the section so I can add a Yakima or Thule kayak stacker. Using the stacker, I can attach 2-4 sea kayaks on their sides and several SUPs stacked on their deck or hulls on the other side.
Before each trip, I do a shake test with the rack bar before adding gear to make sure it's 100% secure. On large loads I'll throw two straps over the entire load and run them through the doors as a backup. Note: Yakima kayak stackers do work on Thule bars if the plastic tabs are removed on the square tie-down sections. I add additional pipe insulation on the kayak stacker pipe for extra protection.