I think you are doing it at a good age, dont have experience of twins but we flew to Australia in November with a 8 month old and a 2 year old, the 2 year old was a lot harder worker than the 8 month old. Mainly as the 2 year old wanted to walk around and was very restless during the flight, there is a limited amount that 6 months old can do.
From what I can gather some airlines are really helpfull and others are not so helpfull. We flew with Singapore and they were brilliant. Buggys are extra to your baggage allowance, we were allowed to take it to the door and it was waiting for us just as we got off the plane at Singapore and Sydney. We used the cartons of milk for the 8 month old and got the stewards to open them for us.
We gave Medised to the 8 month old, but she was fine, it was her sister that was the nightmare.
Take loads of nappies, and I would suggest taking a small toilet bag with a couple of nappies and wet wipes/cream in etc. The toilets are not large, you probably wouldnt have space ofr a large nappy bag, you and baby.
We had a delay coming home of 5 hours in Singapore, we booked into the transit hotel actually in the airport, was very cheap and was great. We could stretch out, have a shower etc. You book them for periods of 6 hours.
Lots of toys, probably quiet toys unless you want everybody looking at you. We had a balloon given to us at Singapore and it kept out 8 month old quiet for hours, god knows what would have happened if she had popped it.....
Good luck and enjoy.