IIRC the sky couch is for two, so the younger kid must be booked as a lap infant. To me, this would imply that he/she cannot be assigned a seat and therefore you normally cannot use a car seat. Also, if you try to use the car seat in husband's seat (across the asile from the skycouuch), this would probably violate rules about carseats not impeding emergency access, usually meaning that the carseat must be installed in a window seat.