About a thousand miles, no problems whatsoever. Used cut-up swimming noodles
for padding and slip straps (the kind with a cam lock, no hooks) and a
little additional rope.
I tied the two down as I always do a single:

1. A single strap down to the tow eye in the front (but in this case had to
tie off the second kayak in the holes below the driver in the front body
area, since there is no tow eye on that side). A second, driver's side bolt
on loop or eye would be nice if this were to be done regularly.

2. Two straps on each boat in the back, one to each eye.

3. Enough strap to go around as many boats as there are and to the luggage
rack hooks to prevent the boats from shifting and provide backup security if
one of the front or back straps should loosen. Easily accessed by climbing
up with the doors open.

A ladder would make this easier but isn't necessary.

I always put all knots and fasteners at eye level so I can see them out the
windshield and in the mirror.

I got about 15 less miles per tank (down to about 335) in the diesel with
both boats up there.

I was driving either in calm conditions or driving rain with a crosswind,
which was noticeable but not too scary, so I don't know what moderate winds
would be like.

I loaded the boats by putting the bow over the back of the poptop, picking
up the stern and ramping them over the top, when it was just me. With help,
we just picked them up and placed them up top. Then climbed the sides/back
to put the foam under before strapping down.

Jim