Harnessed car seat for 8 yo??

I posted a question on here a week ago about putting two 8 yo boys I babysit in HB boosters because they can't sit right when using the adult seat belt. People suggested I just get them both LB boosters that have the belt positioner. I had to watch the boys this past weekend and thought I would have them both right away( different families) and was going to take them to Walmart to pick out which pattern of the LB boosters they each wanted. Well that didn't work because I didn't have them both right away so I didn't want to make 2 trips to Walmart. Anyways to make a long story short. I have the original Britax Frontier in my car for a 4 yo I babysit and one of the boys asked to sit in it when we were going somewhere. Of course I said yes and helped him buckle himself in it. Well, ALL weekend both of they boys were fighting over the seat and who got to sit in it. I took them to Babies R Us today and told them that in the future whenever they are riding in my car they will have to ride in LB booster seats. They were totally cool with it when I thought they would be totally against it! Then one of them said "So does that mean I can ride in the one already in the car?'', and I said yes then the other boy said "I want one like that to, with the baby harness and comfy head rest!". They both like being up higher and being able to see out the window better. They also like the side wings and head wings and surprisingly enough they actually like the 5-point harness. So I'm looking to buy another seat. At Babies R Us they have the Frontier 85 so I wanted to see how much more growing room they had in that one than the one I already have. They do have a lot more in the 85. My Frontier overall fits both of them great, I just don't know what seat to get since they are kind of tall (both about 52"). I also don't know if it'd be best to just get a HB booster and have one seat in that and one in the Frontier. I had them both sit in the Nautilus also and they fit great except that the crotch buckle isn't forward enough. If only it had 3 buckle spots rather than 2! They both also have a lot of growing room in it. One problem is that Nautilus has a 65 pound harness and one of the boys is 63 pounds. The other boy that is barely 51 pounds said he really likes the Nautilus and that it feels fine (he's skinner). So should I get a Nautilus for the 51 pound boy and keep the other boy in my Frontier? The Regent is also an option since I have a friend that is about to get rid of hers. It only seats to 53" inches though which isn't horrible since that's what my Frontier does but it also doesn't have the side wings or convert into a booster. Are there any seats that harness to a high weight and height that I don't know about?!

So basically my options are:

1 boy in Frontier and 1 in HB booster
1 boy in Frontier and 1 in Frontier 85
1 boy in Frontier and 1 in Nautilus
1 boy in Frontier and 1 in Regent

Which sounds best?

The boys stats are:
C- 8 yo, 52", 63 lbs
J- 8 yo, 52", 51 lbs

If you're willing to spend the money, get a Monterey- nice headwings, surrounding cushiness like the Frontier.

My 9yo just outgrew her last harness (a Nautilus), and greatly misses the harness, but really didn't like sitting in a no back booster.

Yeah I am considering the Monterey but would like to spend a little less money. I really like the Parkway and it fits both boys nicely and they have a lot of growing room in it. Any comments on it?

Parkway is a nice booster if that fits them. If you go to the carseatblog page you should be able to find a review for it.

I have an older style and it has surved my DS and my niece very well.

I wouldn't get a harness for kids that big who aren't yours... explain that you need the harnessed seats for other kids, but that there are certainly seats with headwings (we're used to most 8yr olds, especially ones not used to seats, protesting all this... our recommendations for nbbs were to try to help you pick battles)

I bet they would fit great in Recaro Proboosters/Vivos or the parkway SG/SGL

I'll look more at the Parkway.

I know that. But the boys had the complete opposite reaction that I thought they would have and the parents are fine with whatever I do since it's my car. I know you don't NEED to harness an 8 yo but the way they sit now in LB boosters is like almost not using the seat belt at all so they are a lot safer in HB boosters or harnessed seats. And it's not a battle to get them to possibly use a 5-point harness, they requested it.