We just got a SMB with a penthouse roof and Yakama bars and took it to the North Coast ab diving with 2 Ocean Kayak Scupper Pros on the top.
We tye them down upside down, with lash straps going thru at least one scupper hole per boat.
We use a bow line "just-in-case".
We can just barely lift the top with them up there, and I wouldn't trust the top up with any wind with them up there.
They also seem to bow the PH top down a little, like they are too heavy for the top @ ~130 lbs for the 2 of them.
They are also a PITA to get up and down and to tye down, I had to buy a step ladder.