To be totally honest it's a pain to change car seats between cars all the time once you get past the baby stage.
It can be done, but it's annoying and heavy and the more you're installing and uninstalling something, the more chance you have to make a mistake with the installation.
Is it for budget reasons? Do you have a total budget in mind to spend? It might be better to get 2x cheaper seats and leave one in each car than move it every time, unless it's only occasionally going to be moved.
Also, how close will the cars be when you're moving the seats - can you have them pretty much side by side on a driveway/street, or will it be a case of removing the seat, storing it in the house, taking it out down a flight of stairs, carrying across a full car park in the rain etc. (Think about all seasons/weathers/times of day for this and whether you'll have your toddler with you).
Isofix seats are much quicker and easier to install, but they tend to be very heavy for this age group so it's not too bad if you are just swapping over a short distance but if you needed to carry the seat any distance at all, isofix is a pain because of the weight. Axkid One is probably the exception to that but it's very expensive, more expensive than buying a seat for each car.
Or if you already have 2x bases and they can take a toddler seat, that might be worth looking at (but does mean if you have another baby, you can't then use the bases for the infant carrier because the toddler base compatible seats can't be seatbelt fitted.)
The Joie Every Stage R129 (or Graco Slimfit R129) is a good option if you are really keen to do it this way. Once it's installed, you don't need to do the seatbelt again but do be aware that the seatbelt will be in the way of getting the child in and out, which is a bit annoying on a seat you'll be using day to day.
You can see the installation process here (it's the older R44 version of the seat, but the newer one is almost exactly the same, just green guides instead of red)
Another option to think about if it's the same 2 cars every time is looking at one of the extended rear facing seats which go up to 125cm and just getting 2 sets of tethers, so you only need to move the seat itself and then click into the tethers rather than rethread them every time. That has the advantage of being lighter than an isofix seat and the belt routing is simpler and less in the way than a purely belt fitted seat. (As well as lasting you longer).
Do you know roughly if your LO is big/small/average sized for their age?