: I've carried two doubles on a rack with shorter
: spacing than that, and I'll admit it was frightening

Ah, knew I had a photo somewhere. A quick measure suggests that the
spacing between bars was well under 30 inches - no wonder it felt
wobbly ! But, as I said, nothing went wrong with this arrangement
(there was a tie-down added at the back before driving off). On
this occasion the singles were on the other car as we really didn't
trust this rack to take all four boats. With the home-made rack,
the front bar was able to move forward and the back bar back a bit,
but I'd estimate that the spacing was still less than your 43", and
that happily carried two doubles and two singles at high speeds.

Andy