My brother and I double deck our yaks all the time. He has a PA 12 and I peddle an outback. his goes on first keel down and mine goes on top keel up. Each are strapped down individually to the rack. We drive 180-250 miles round trip depending on where we go and have never had a problem. If I can find a pic I will post one up.
I drive a Suburban and he drives a wrangler we have done it on both, carrying the yaks I mean.