Trouser size is less important than top size, what size tops is she in and do you know how tall she is and her weight? (Also current age with months would be helpful).
The Joie 360 is one of the less-awful multi stage seats for newborns but it's still not ideal, you have to be very confident in fitting them, and you will lose that carry out factor as you said.
The Joie 360 is pretty enormous and is very good for long use - I have a big 5yo who could have stayed in it until quite recently, though had moved him out for his brother to have the seat. Children seem to outgrow it somewhere between 110-115cm depending on whether they have longer legs or body. So in terms of money saving, it might make sense to go with the biggest baby seat you can find (Cybex or Britax, usually - both have basic or premium models) and try to squeeze as long out of the Joie for DD1 as possible. OTOH, even if you did stick with it for a year, just turned 4 is on the young side for moving to a high backed booster seat. So it might make sense to look at a larger seat for DD1, whether you move her ASAP or go for the baby carrier first.
Is DD1 rear facing and if so would you like to continue? The leg room on the Joie 360 rear facing is totally crap. So if you did want to continue rear facing, it might be worth looking at the longer lasting rear facing seats like Axkid Move, as they have much better leg room and the extra capacity (up to approx 7 years) just means you don't need to worry about how long it will accommodate her. Whether you want to move her out at 4, 5, 6 ... totally your choice. And if DD2 turns out to be even taller then it future proofs you too.
OTOH if she is already forward facing or you think you will move to FF soon, then it might also be worth looking at a seat which is designed specifically for forward facing. The Joie 360 Spin has a good safety rating, but some of this will be afforded to it by the rear facing mode, as safety rating is an average of all modes unless one totally fails. The spin places a child higher and further forward in the car, which is less protective than a seat which places them lower down and further back. For example you could look at Britax Evolvafix or Advansafix Pro, or Maxi Cosi Titan i-size or i-size Pro. Silver Cross Balance i-size is also (I believe) equivalent to ABC Design Aspen which received a good safety rating. The Britax and Silver Cross are both 105cm/21kg weight limit on the harness. Maxi Cosi is 105cm/18kg.
Joie Bold is worth looking at if you are likely to need a longer lasting harness, though the safety rating on it is not as good, so I wouldn't choose it if you don't need the extended harness. (If you do want this it's advisable to buy ASAP even if you store it for later, as it's been discontinued and the newer version doesn't have the high weight limit.)