If they both fit the rack upside down that is a start.
What kind of rack padding to protect the decks and prevent sliding side to side?
You need front and back tie downs.
Use ratchet straps carefully. You can easily deform the boats.
Do test rides at highway speeds.
Beware of high cross winds.
I agree with string. Pad the crossbars with split foam pipe insulation or split pool noodles. I don’t recommend ratchet straps. Too easy to overtighten and damage the boats. Poly cam straps are more than sufficient. They don’t stretch, are very strong, and I’ve never had any loosen up even after 1,125 mile trips. With bow and stern lines they only need to be snug.
Bow lines at least.
Consider regular straps, but for that distance I would use two at each point where most people use one.
And there will be some noise. Turn up the radio. Paddling is worth it.
Hull up if it rains. Ok to have one resting on the other at angle. Tie down the flat one first, then the one resting on the first one so they both fit on your crossbars.
J bars are no good. Use them once and you’ll learn why.
Your basic “J” cradles are traditional for Rec kayaks. Use Bow Lines for sure, keep your speed down, and check the load every so often.
Now if you don’t want to buy, or can’t find J cradles you could use 'U" bolts to mount 2X6 X 72" boards to the factory cross bars & then strap the kayaks deck down to those.
I wasn’t thinking of keeping them clean, I was suggesting them to go over the cockpit side to keep the wind from getting inside the kayaks, keeping the noise down. If you’re just looking for tie-down suggestions, like others have said, J cradles, cam straps, and bow and stern lines.
I would extend the crossbars with 2x4s 68” long. That leaves 4” in the middle and 4” on each side. I put some on my Kia Soul that are 88” if I recall as I want to be able to haul 2 full size canoes at highway speeds.
If it were mine I would drill thru your current cross beams at 2 places each and hold the 2x4s down with 5/16 bolts and nuts with the holes in the 2x4s counter bored for the bolt head and a washer, leaving the top flush for sliding the boats. If you fear drilling thru the cross bars then something like a U-bolt would work. I would make the U-bolts from flat bars with a hole in each end for bolts. If you do drill thru the cross bars get a shorter bolt to put thru the bar when not using the extensions to keep wind noise away.
You have a big long car and shorter boats. You will be using two loop cam straps around each boat and they need to double under the cross bar over the hull doubled and then loop over the extension 2x4s. I will post a couple photos. Putting bow and stern tie downs on might be a problem because your roof is pretty long. They sell or you can make loops that fit under the hood and stick out the crack to attach the lines to. I use loop cam straps here also some people use rope. I will post a photo of what you can make to slam your lift gate on to provide rear tie down spots.
Lastly if your extensions stick out past the car round over the corners and paint the ends a bright color to keep from banging your head. Your car may be tall enough that’s not a problem.
J racks will work but are a pain to load and unload on a high car. Keeping the boats hull up you wont have to worry about water and IMO they are most aerodynamic that way.
I had a rowboat once which i carried ontop a wood utility trailer
i positioned the rowboat ontop the trailer. and drilled 1/2 “ holes in the trailer top surface of front and rear walls at the rowboat points of contact 4x
If either boat is carried cockpit up and it might rain, you should use a cockpit cover. Just a few inches of rain in the boat can add a lot of weight and can cause a bit of strain as the water shifts about. it can also reduce wind resistance and noise.
If using a cockpit cover make sure that it is secured by something other than just the cockpit rim.