There are quite a few companies that do this now :)
Maxi Cosi - Familyfix system - I'd strongly recommend getting the one which allows rear facing past the baby stage, and be aware their infant seats in this range are very limited in height limit, meaning the infant has to go into the toddler seat before 1 year old which can be very upright for them. I recommend an infant seat which has a height limit of over 80cm if you're looking at i-size ones, and Maxi Cosi don't make any currently.
Cybex - their range with "M" on the end (stands for Modular) - The only one that offers a spin seat in this range currently, and the only option for a spin interface for infant seats, but their infant seats are heavy, especially the cloud (ironic name!) Some people report a problem with the recline mechanism, but this is covered by warranty, and may be fixed in newer models.
Axkid Modufix system - best legroom and height for rear facing toddler seat in this category, useful if you/DH are tall and slim and likely to have a tall, slim toddler. Infant seat needs an in-between adapter for pushchair use.
Recaro Zero one Elite - works slightly differently - the infant seat clicks into the bigger seat. Can be fiddly to use due to the height, but racing fan/petrolhead DHs tend to love the brand.
Joie - their seats with "i-" in the name (their i-size seats) - a couple of the infant seats (e.g. i-level) don't work without isofix base. Tend to be the best value buy, but shop around. Some people find the i-gemm isn't great for a newborn, I'm not sure if the i-snug is better. They are supposed to be bringing out a better insert to help with the i-gemm.
Venicci Ultralite + Aerofix - Not popular/well known in the UK but very good - most lightweight infant carrier in this category, excellent positioning for newborns.
Besafe izi Modular - One of the original purveyors of a modular system. Highly respected in the rear facing world. Nice, high quality seats.
Happily all of these are compatible with the most popular pushchair adapters. If a pushchair says it takes a Cybex, Maxi Cosi or Joie car seat, it will take any of the infant seats in these systems.
Notable by their absence is Britax. Britax are one of the top makes for safety (they helped develop isofix, for example, now a worldwide industry standard) so I tend to take notice when they don't participate in what seems like a popular trend in the car seat world. But it may be more that they don't consider it a priority more than that they feel it's unsafe.
However yes I would consider the practicalities of it. It does sound like a nice option at this stage, but be aware the car seat companies think it sounds like a great option too, because it means they have you as a guaranteed customer for the stage 2 seat which tends to be quite an expensive one. You might want to have more choice when that time comes around and being restricted to one brand can rule some options out which you might want closer to the time, for example a spin seat or a seat that goes up to 25kg (larger than average children can outgrow standard 18kg/105cm toddler seats by age 3 or even 2.5).
Add up the cost of infant seat + base x2 + toddler seat and compare to the cost of infant seat + potentially cheaper base + separate toddler seat (including 25kg options). As said above most toddler seats do not require a separate base, though the modular ones are easier to move between cars if both have a base set up. If you're planning to move between, your idea might be best. If you want a seat in each car permanently due to split nursery pick ups for example, infant seat + 2x non-modular base + 2x non-modular toddler seats tends to work out cheaper.
If you have two children within 3-4 years you may want to reuse the baby seat, which means using it without base or buying a second set of bases for the new baby, so add this into costs if you think you might do this. Alternatively if there is 2-3+ years gap you may then simply buy a larger seat for the elder, so it can work this way as well.