M.J.
I carried the carseats on and buckled the boys right up! My boys are 18 months apart and I felt it was safer to have them seatbelted in just the same as in a car. The airline folks were more than helpful when they saw me with two boys and two car seats. (I'd take a stroller and use it to transport the seats and let the boys walk.) We travelled to Canada, Japan, and across the USA this way. It was much easier than you might think. And it taught my boys how to sit still on an airplane. Did I mention, they would nap in their car seats?