So I know the shoulder belt fit properly when we did that. Does weight matter? Is a heavier 7 year old better off in the backless than a lighter 8 year old?
Belt fit is more important than the actual weight or the age. If the belt fit is terrible (say on a Cosco combo seat used as a booster) but is GREAT on a Harmony Literider (which is backless), we're going to tell you to use the Harmony whether that child is 7 or 8 and weighs 50 or 70 lbs. If there's no headrest in the vehicle and the tops of the child's ears are going over the seat back, we're going to suggest a highback booster, no matter what, if possible.
If the belt fit is great on both seats, hbb AND nb, I'd put the older child in the no back and the younger one in the highback. Weight would be secondary to the age. It makes sense that more body mass might predict a better outcome (heavier bone structure), but the difference b/t a 50 and 60 lbs child at that age is quite negligible.
Any time it's possible or reasonable to use the highback, I think it provides additional protection, but I understand at some point, kids may not want the highback. I think 8 years old is a very reasonable age to let go of the highback. I've even let my 6 yr old ride in a backless occasionally. We have a Harmony nbb to use for friends or for him when needed.
henrietta