OK so your gap is around 13mo. Your boy is around average size so you can estimate he will outgrow an 18kg seat by around 4yo, especially the Joie 360, which has a nice long tall back.
You're expecting a girl who tend to be slightly smaller. You have two generously sized infant carriers (is it the R44 cabriofix, or the i-size one?). It's actually possible she could use these seats (at least the Cybex, but also the Cabriofix if it's the older one) until she is about 2.
At this point your son will be around 3, so can't move to a booster seat, he will need to continue to be harnessed, ideally rear facing. So something like Axkid Move would be great at this point. You could also look at something like Joie Bold, or Britax Evolvafix depending on your preference at this time and his stats.
Assuming you go the ERF route - so the kids are 2 and 3 and move up a seat each. Eldest in Axkid, youngest into the Joie.
It's likely the Axkid will last your eldest until he's around 6-7 years old. Your youngest could use the Joie until somewhere between 4-5 depending on how tall she is. It is unlikely she'll outgrow it before 4, unless she turns out to be very tall. Eldest will be 5 or so. Will you be happy to move to HBB at that age? If so then this is a good combo. In fact, if you stick to long-backed seats and your DC are average height or less, then you could get away with 2x 18kg seats if you are happy with HBB at 4.5 or so.
If you want to be sure you can max out both DC in ERF seats, then it makes sense to go for 2x ERF seats now. However check the suitability - the R44 Axkid Move is too big for a 72cm tall child, children typically fit better by around 80cm. It might be that by September, he's fine in it, but it's a bit of a gamble. The Movekid adjusts a good bit smaller so is likely fine even from 72cm.
If you are less bothered about maximising rear facing time and just want to get to the 4 year milestone, I think it's highly likely you'll be able to do this with an 18kg + ERF combo. You may even be able to manage with 2x 18kg seats, though this will be more of a gamble. (You can always wait until your youngest is getting close to the infant carrier limits to judge this).
One thing to bear in mind with the Joie seat is that the leg room is severely compromised RF after about 2 years old. It works, and you absolutely can have a child RF up to age 4+ in it safely, but if for example your partner is not totally sold on ERF it can cause frustration and the fact you can easily just turn the seat around means that a lot of people do end up doing that.
For a slightly higher price you could compare with Cybex Sirona Gi i-size and Britax M Plus - these also both have a higher weight limit at 20kg which might help extend the life of the seat for you slightly. Because they have a rebound bar, the leg room is a little bit bigger in these seats. John Lewis or a local independent baby store would be good places to go if you want to have a look and compare the three.
Bear in mind also that the side impact protection on the original Joie 360 Spin is quite minimal compared with other spin seats. You could look at Joie 360 Spin GT or Graco Turn2Me i-size (which is based on the GT version) for some similar seats with more up to date side impact protection.