There’s about 51 inches of room from inside door to inside door.
I have two Britax romers and there is about 10 inches between them. As there fixed in location by the isofix mounts.
A very tight squeeze for an adult.
I understand some child seats are slimmer but still doesn’t leave room for adult in middle. Whenever we have to carry a 2nd adult we have one child in the front in a car seat when legally able to do so. Leaving one child and two adults in rear of car. An option maybe.