I have a set of Firestone RideRites on my F250 and have the sides plumbed together. I don't experience any extra rolling on turns. I think having them plumbed separately is important mostly for truck campers which tend to roll a lot. With a center-point hitch like my trailer uses, I don't have any problems with them being plumbed together.
Relative to the psi variations with weight, I think some of the early posters were thinking that weight doesn't seem to affect tire pressure. That may be relatively true, but there's a lot of differences between tires and suspension air bags. An air bag doesn't have the stiff sidewalls of a tire, so significant addition weight on air suspension airbags does cause significant pressure increases compared to a tire.