"I want the ride height to be leveled when there are 2 in the car,not the back lower"
Won't this mean that when you are driving without a passenger, the back end will be slightly higher?
You won't want to be constantly adjusting the spring seat platforms to compensate, so I think what you are really looking for are a pair of coil springs which get harder the more weight that is placed on them. I can't remember the actual description of them right now, but I know that Caterham used them on the back for the same reason.
I'll post back when I get my mind into gear.
