I've just posted this in Travel:
www.mumsnet.com/Talk/shorthaul/1130943-Top-tips-Madeira-with-20mo-twins
and here are some addition points about travelling with twins.
Aeroplanes (some? all? not sure) have a limited number of infant oxygen masks and there is only 1 per row where they exist. Result? DT1 crying all the way through takeoff and landing because although she was happy to sit on Granny's lap, she couldn't see Mummy. Mummy, with wriggly DT1 on lap, heart breaking 2 rows away. It was fine because the plane was half empty and we moved after takeoff but beware of this on a full flight and have at least toys, drink and snacks in 2 separate bags, if not a full kit. I didn't think it was worth paying for seats but with hindsight the advantage of having 2 could have been that we could have paid for just 1 extra seat (and more importantly one extra baggage allowance) so that's worth bearing in mind if it's not prohibitively expensive. We were very overweight with luggage, which again, was fine because the plane was so empty, but might be expensive in a different situation.
The best things I took were travel Magnadoodles and many, many small boxes of raisins. I also really recommend the travel booster seats instead of high chairs.
We took 2 single strollers and I bought stroller connectors (Prince Lionheart make them, buy them from Safetots). You wouldn't want to push the 'double' very far, but it was quite useful for moments where there were fewer hands available, and especially when they wanted to walk - it meant we had only one buggy to worry about instead of 2 and as there were 3 of us, one could have the buggies, and the other 2 a child each.
Hope that's helpful to someone. Perhaps others might add their tips to make this a useful thread?