Weight for Group 2 car seat (high backed booster)

[PrincessOfWails]

OK, so, the weight is supposed to be 15kg.
My dilemma is this though - why 15kg?

Reason I'm asking: I have 2 DS. DS1 is over 4, and DS2 is 13m. I had hoped to avoid buying two of the same kind of car seat (and thus save some ££ as they're expensive) by moving DS1 into a high-backed booster, and DS2 into the forward facing group 1 car seat.

The problem is that DS2 is fine on weight and age for the car seat; DS1 is fine on weight, but the car seat says 15kg; he is 14.1kg.
Plus DS2 is really annoying difficult about going into his rear-facing one and will contort and scream etc!

What should I do? Shift them both up, or wait? I could have a long weight for skinny-minny DS1 who isn't heavy and grows quite slowly afaik (same size feet for over a year now!)

Thanks!

I wouldnt put your ds into a seat that he is too light for as it may not protect him in the event of an accident. The second stage car seat is ok up to 18kgs which is the weight I moved ds into a high backed booster. You could get a group 123 seat for your DS2 and then do a swap when your ds1 is heavy enough for the group 2 stage.

I would get a group 123 booster seat with a harness for DS1, he'll use it with a 5 point harness to start with, then when he is eventually heavy enough you can remove the harness and use it with a seatbelt. That way you won't have to buy any extra seats that won't last long, your DS1 can be in a 123 seat until he is 12.
We had to do the same (DD1 at 4 also not heavy enough for a high back booster without harness, but too tall for her group 1 seat)
We got a Britax Evolva 123 for DD1, and I am very happy with it. Bit more expensive than a standard group 2 seat, but at least it will last a long time. I'd heard it is particularly dangerous to move children from a 5 point harness before they are big enough, so this was the safest, most sensible option for us.

When we were in this position we got a Group 1-2-3 (Recarro Young Sport), moved DS1 (then 3) into the new seat and DS2 (then 14 months) into the group 1 seat. Has worked fine, and has meant no more seat moves - if we'd but DS2 into the 1-2-3, then we'd still have had to moved DS1 into a booster at some point (either buy buying a new booster or swapping the boys seats), but this way I only had to swap DS1 from the 5 point harness to the adult seatbelt when he reached 18kg, and both boys got to keep "their" seats. I'm hoping that by the time DS2 needs a booster we'll be able to buy a fairly cheap one, i.e. a simple booster, not a 1-2-3.

Think this is exactly what Maamekin is saying too.

well you should keep your little one rear-facing for as long as possible because it's far, far safer than having him forward facing.
so personally i would wait

Yes, thanks everyone! I already have the high backed booster (got it as a gift) so buying another isn't an option really as I'm unemployed now.
But I was wary about risking it, and the plan B was to keep things as they are, with DS2 in the rear facing, notwithstanding the tempers! I'll weigh DS2 again in a couple of months.