I think you mean ..............and SUP.
That New Scotty chap is funny isn't he?
Seriously though I transport a 5.10 m sea kayak and two SUP's. the SUP's sit on top of each other one is a 12'6" Fanatic Fly and the other a 9'6" All Wave.
The kayak sits in a kayak cradle which is positioned as far to the outside as possible. You have to make sure you are carefull with your straps as they have to go back under the post to make sure they are secure.
The SUP's then take up the rest of the space on the other side. My racks are Rhino but all the accessories are similar.
I am guessing you must have a smallish car if you are looking at one on top of the other.
I would experiment with it before you go any where. I would also tie each craft down separately that way if the top one comes away the bottom one is still secure.
Also don't put too much pressure on your SUP when you tie it down, they are strong but tight straps will mark them.
Hope this is of help. What sort of car do you have?
ET.