I have this set to carry two 12’ kayaks that seems decently stable. Just took it on a trip an hour and a half away on Friday.
One thing I would warn about with this type of mount is that since the kayaks are held relatively upright, you may want to make sure you strap the first kayak down (or bungee it like I do) to prevent it from falling off from the bump when you load the other one. I broke a side-view mirror off that way.
I bought this from amazon and it has been great for the past two seasons. Easy to get on and off, secure, does not damage my yaks. It has no problem with two kayaks (driven over 500 miles with kayaks on the car). My only qualm is that the bolts it comes with are a bit long and take a while to screw on but you can always cut them down. A bit more money that what you're posting but still less than a Thule