The general recommendation is that a preemie/infant be removed from their car seat MINIMALLY every 3 hours for at LEAST 30 minutes.
I would highly suggest that someone ride in the back seat with them if possible because sometimes preemies can have 'issues' when they are in a car seat. For example, their heart rate can drop, their oxygen level in their blood can drop, etc. If that happens while you are driving, you need to pull over immediately, stop the car, and quickly remove the child from the seat.
I would suggest, if possible, that you actually put your babies in their car seats for a couple of hours to 'test' that the babies can tolerate being in the seats. If they are at hospital, it would be best to do this 'test' while the babies are attached to a heart monitor...otherwise you will need to sit there and watch them to make sure they don't have any issues.
It will be very important that you make sure the car seat is installed at the correct angle...the seat should have some sort of angle indicator on it. The seat should not be too upright, but it's also not safe for it to be over-reclined.
I'm not familiar with the car seat you mentioned. It's a European seat so I assume you are overseas, correct?
Does that help at all?