It looks like the only sure way to get a comfortable trip with babies is to book them a seat. Here is what I've discovered..
* You can't rely on the airline providing a bassinet. Let alone two.
* You can't buy your own approved bassinet that will clip onto
the bulkhead or elsewhere.
* The bulkheads in Business class are not designed to take
bassinets on some airlines even though they may offer them
when you book.
* You can't have them in a baby carrier at your feet. There's no
room at your feet in economy for a baby carrier anyway.
(Although this might be an option in business class if you pick
them up when the seatbelt sign is on).
For the trip I referred to in my original post getting them seats would have cost us $8000. The business class fare being $4000 at the time and we have twins. The $400 we paid was 10% of the adult fare.
But perhaps I shouldn't complain about this trip too much.. In the end there were two empty seats in Business Class and the passenger that was asked to move was only too glad to escape sitting next to twins for 9 hours.