It probably makes the most sense to get one of the 125cm limit ERF seats for DC1 and then DC2 can inherit the spin seat for now, so you have the extra capacity in case DC2 stays on the higher centiles. OTOH at 9 months, it's not uncommon that they drop centiles when they start walking/running and getting into the toddler fussiness with eating, so it might be that their growth slows down a bit.
As DC1 is older, the spin is less necessary and I would consider a fixed seat as they can have better leg room and tend to be much easier for DC to climb into than the spin seats like Joie.
They can get pricey, so I would look at Britax Safeway M, Britax Max Space Pro, possibly the older model of Avionaut Sky (2.0 or Q) - those are the ones which seem to have discounts currently and can be found under £300. If you have a local physical retailer and can pay cash, it is worth asking for a discount when you're buying two products. Sometimes it is possible.
If you did want a spin seat with RF up to 125cm, Avionaut Stardust could work, but it is more expensive at £349, especially if you need two.
If this is too much then another option could be Britax Dualfix Plus, or Cybex Sirona Gi i-Size as both have a slightly higher weight limit of 20kg as opposed to 18kg on the Joie seat, but they do have the 105cm height limit, which might end up being an issue for your younger child later down the line. These are spin seats and they regularly go on offer for under £250.
Lastly Joie i-Spin 360 or Joie i-Pivot are on offer on amazon for about £175 each, the i-Spin is similar to the original Joie Spin, with a slightly higher weight limit (19kg / 105cm) and similarly tight leg room, whereas i-Pivot has 21kg weight limit / 105cm height, and slightly more leg room because it has an anti-rebound bar.
I am not sure which Joie spin seat you have now, but the original Joie Spin 360 has an 18kg weight limit and children usually fit up to about 110-115cm by height, so it can accommodate slightly taller but less weight wise whereas the i-size versions are limited to 105cm height but have a bit more wiggle room on weight.
Personally I would not look to get a spin seat with high back booster mode because 9/10 the high back booster modes on those seats are not very comfortable and don't give a great belt fit. Aside from the Avionaut, which is worth considering because of the better height limit for RF, I would stick to seats which only cover the RF/integral harness stage. When you get to the high back booster stage later down the line, those seats are less pricey anyway.