Hi Dan,

I just install them with one bolt on each side. Screws work loose, and
bolts with nylocks don't.

As for side to side shimmy, I haven't had that problem. I put the
thwart in to keep the sides of the boat from flexing too much, but I
want them to be able to flex a little. If there is no movement at all,
and you hit something hard, well... somethings gotta give. I'd rather
have a little flex on that joint, than to have the whole thwart rip
out. Besides, putting one hole in the gunwales is less damaging than
putting two holes close together.

Just my $.02