I would... they aren't just siblings that are different sizes, they are twins... and why give them more to compete over than they already have in life?
granted, if they had different favorite colors and you could get covers in each... hey, get them the same seat in two different colors.... just helps tell whose is whose.
but otherwise, matching seats is just plain sense... (of course, I prefer it regardless, which is why my boys had matching turboboosters, then matching Regent and Husky... yes, I ordered a regent cover for the husky so they'd match... and then, after Damian went to booster, Jeffrey and Ruthie had Radian 65's, but with different covers... and if they didn't have totally different builds, they'd be in the same booster with different colors)
OH, and let us not forget the 9yr lifespan would pay off most for the smaller twin... and the frontier IS a super tall booster... I think it makes sense since you can afford it (LUCKY DUCK!!)