i use the yakima stacker with the hooks on the end, they take up very little bar space and can be rotated down very easily to make room for other mounts and allow me to get into parking garages. i am not a fan of the "loop type" stackers, they take up more bar space and just seem inefficient to me.
i have put up to 3 kayaks on one side of the stacker as shown in the other picture attached with no problems. i have figured out a way to cross the straps up to keep the stackers from spreading out at the ends due to the shorter boats (you can kind of see it happening in the picture), but my diesel doesn't have that problem if it is on the rack alone because it is a longer boat. later.