Our twins will be 20-months-old for our next WDW trip. Since the seats are 3 on each side of the aisle, we purchased 3. One baby is booked as a lap baby, and the other has a seat. We've flown twice before with them (7-months and 10-months) and both times we booked it this way. It was worth it to have the extra seat, and not worry about annoying the poor person who got that 3rd seat.
I'm looking at the size of these Marathons and thinking, "How did you guys do this?" Were they difficult to get on the plane (down the aisle)? Did the flight attendants let you board early?
When they were 7-months and 10-months, it was tough boarding. But then again, we had to carry the babies, in addition to diaper bag, 2 infant carriers and carry-on bag (no way I'm checking expensive digital & video cameras). Now they're walking, so at least they can walk down the aisle.
Since we've only purchased 3 seats, we'd only be able to bring 1 Marathon on-board, and would have to check the other. I would definitiely buy a protective travel bag for it, because our Graco Duo Glider got banged up during our last two flights. But that means 1 baby is in the car seat, and the other is "free"...I know whichever one I put in the car seat will not be happy to see the other one "free".
Maybe I'm just better off buying 2 protective travel bags, and checking both of them. After take-off, I imagined the twins could sit together in the extra seat and watch their portable DVD player.