My pick for infant car seats would probably be Avionaut Cosmo. They go on standard pram car seat adapters so should be compatible with any buggy which takes 2x (Maxi Cosi/Cybex type) car seats. Plus they have a great position for newborns and they are lightweight which presumably helps if you ever have to carry two at once (I have not had twins, so I have no idea if that is a thing you have to do).
OR if you can get hold of them Cybex Cloud Z, T or G. (Whichever you can find a good price on). The benefit of this is that it reclines, at least on the buggy - so it is more suitable for using as a buggy if you're out for the day. Technically you are not supposed to use car seats on buggies for too long because it's not great for the baby's airways.
For both of these you would be looking at seatbelt fit, but it doesn't take long. I do prefer the Avionaut Cosmo to the Pixel, because the seatbelt fitting is much easier. The Pixel is fiddly.
The only infant car seat which can be fitted with isofix without needing a base is the Nuna URBN or Joie Sprint, and the problem with both of these is that they are max 75cm, which is rubbish, it's outgrown within a year. Maybe a little bit longer with twins as they tend to be smaller, but still. Then you will be stuck because in modern car seats they have to be at least 15 months minimum to go forward facing (and it's better to wait as long as possible).
With the Avionaut or Cybex the height limit is a massive 87cm, which is more like 2+ years. Realistically, you are unlikely to use it that long although you absolutely could if it is still working for you. You have the option as long as the baby's head still fits within the shell of the seat. Legs are allowed to be off the end.
For the next stage it is likely you'll be limited to FF seats (Maxi Cosi Nomad or Cosatto On the Go) but you could look at Nachfolger or Tinyseats Two if you don't mind spending a lot more to get rear facing.