We went on a 6 week road trip across the US followed by 2 weeks in New York and 3 weeks in France - so total of 3 months - with our DTs when they were 6.5 to 9.5 months.
I have posted the notes I made on travelling with twins below. Basically you need to analyse every activity you do and try and minimise the stuff you need for it - also remember that you can buy stuff out there!
Best time to travel - Looking back, I think the best time to travel with babies is before they start solids - some time between 3 and 6 months. At this time you have your routine established, babies will stay where you put them and as long as they are clean, fed and rested they are happy. After 6 months you have to deal with solids (both the hassle of feeding and the change in nappy contents..yuk! and then they get more mobile and more opinionated. We travelled when the boys were 6.5 to 9.5 months - and it has definitely got harder as they have grown.
Aeroplane - The UK airports don't allow pre-prepared milk through security. But we found that Boots air-side always has a stock (but take a powder back-up). The US had no problem with our taking pre-prepared milk on board.
Sterilising Bottles - When the boys were 3 months we went to Budapest for the weekend and took enough disposable Steri-bottles and pre-prepared formula to last our trip. Obviously that was not an option for a two month trip! We took 1.5 days worth of bottles with us to give us a little breather on washing and sterilising them. Before we left I was already feeling that sterilising was a waste of time but could quite give up. So I decided that I would just do the teats on our trip and that a good wash in very hot soapy water would be sufficient for the bottles. I had already tried the disposable cold water steriliser bags on the Budapest trip and found them a nightmare - they are unwieldy and will bleach anything you spill the liquid on. Knowing that some of the places we were staying would have a microwave, I bought a little box from Boots for about a tenner that doubles as a cold water and microwave steriliser.
But after a DT sucked DH's Elk poo covered shoes, I decided that the whole sterilising thing was stupid and I stopped - the boys do not live in a Howard Hughes environment - they suck toys after the other one has vomited on it! Obviously if you are breast feeding you won't have any of these issues. But travelling will take longer as you can't breastfeed on the go in the car.
Formula - In London it's really easy to buy pre-prepared formula and it's really cheap so I thought that we would just buy it as we went along to save us worrying about making it up and to save time but in the US it was not so so easy to find and is about four times the price - so, in thjose few shops where pre-prepared milk was available, we tended to by ou the whole stock. In the US it comes in a ring-pull tin can and is much heavier than the cartons in the UK - there is also only one make (Enfamil) that comes in individual pre-prepared cans. In France, again there is only one make that seems to do pre-prepared (Quigoz) and it comes in 500ml cartons - good for travelling twins but of little use for singletons.
Solids - I had daydreams of doing baby-led weaning and getting them to eat whatever we ate on the road... silly me. When you are travelling and you are already doing four milk feeds a day you just need to get meal times over as quickly and cleanly as possible (to preserve laundry and also because you are on holiday and who wants to spend hours feeding twins?). Also we were eating rubbish most of the time especially in the Wild West when the only food available was hamburgers and steak - hardly suitable for babies. So I swallowed my pride and fed my kids from jars of manufactured puree etc - plenty of time for proper food when we get back home. We have given them some finger food - cream cheese on a bagel was on many a hotel breakfast menu as were chunks of melon which you can just stick on a fork and give to them. One DT loves his finger food and the other loves the jars - just my luck to get one of each. Banana would have been great and they love it but it gives one of the DTs a face rash. In France they have enjoyed natural yogurt mixed in with fruit puree and wheetabix for breakfast, and they love to play with rice cakes and bread at restaurants when DH and I have our lunch.
Disposable bibs - These are essential on the road - saves on the laundry- who has time to wash bibs?
Clothes - We took too many clothes. Take enough for 7 - 10 days and wash as you go. Most places we stayed in the US had a guest washing machine and dryer - so laundry was really easy. Also as you travel with twins your dirt threshold goes up and clothes that at home would have gone straight in the wash are fine for another outing (or two). DT and I got used to spending our days in baby-milk/sick/food stained clothes - embarrassing in some of the posher places we stayed.
Car - If you hire a car, get one with space between the two back seats so that one of you can sit between them and entertain and feed the kids on the road. There is nothing worse than having two screaming babies in the backseat as you are driving along with no way of soothing them.
Nappy changing - The boot of a people-carrier/4x4 is an ideal baby-changing table. You can also use the passenger seat of a regular car though the stooping kills your back. Secluded spots in parks are also good. The US was great as most places have baby-changing tables in both the men's and women's toilets so we could save time by doing one each. We have not seen one baby-changing table in France apart from in the airport. Oddly it was not as easy to get nappy sacks in both the US and France as it in London and so we have been hoarding plastic carrier bags and using those to save our stock (in Canada we bought dog poop-a-scoop bags!).
Sleeping - The boys slept in 12 different beds on this trip - it really is amazing how flexible they were and how easy it has generally been to get them to sleep - even in places where we have all shared a room and in the evenings DH and I watched DVDs, had takeaways and chatted while the boys just slept. Most places provided a travel cot with no sheet - so it's worth buying a couple of travel cot sheets and taking them along to give the children some luxury and comfort.
Buggy - We are so glad that we took our good buggy with us and not our cheap "umbrella" one; - we could take it off-road (perfect for the beach and Yellowstone Park) and could lie it flat and put the boys to sleep in it when we went out to dinner (with a travel rug over it to shut out the light and prying eyes - it's amazing how many people want to poke a sleeping baby!). This meant that we could go out to dinner in the evening when there were restaurants nearby. This is another reason to travel with babies before they get too big to cart around asleep. I wish I had taken along a spare inner tube for the buggy tyres - we had a puncture in France and it was really hard to get it fixed. A flat tyre has the potential to ruin a holiday. We also used the buggy as a highchair.
Toys - We took a few and bought a few on the way.
Baby Bjorns - These are not comfortable for long term use when babies get big - but they were very useful when we went on small boats (whale watching), and buses (it's easier to fold up the double buggy if you have baby in a sling). Also in Heathrow you have to pick up the buggy at the carousel with the other luggage - so Baby Bjorns are useful for carrying baby as well as hand luggage on the long walk to baggage collection. (In Vancouver we travelled from the aircraft to luggage collection on one of those electric carts reserved for disabled people, which was great.)
Muslins - A great invention - both for clearing up sick and other mess and for comforting the boys. When they are yelling from tiredness, you can get them to shut up just by sticking a muslin in their face - amazing! Definitely a dependency to encourage! We also used dummies to comfort the babies. I hope that this is a habit we can break when we get back home but we would not have survived without them on the trip.
First Aid - I made up my own first aid bag with things that I knew would be useful. Metanium is the best nappy rash cream in the world.
Blankets rather than coats - Easier to put on and off when you go in and out of buildings and cars - with twins the whole putting on and taking off layers is a real pain. I am sure all multiple parents have noticed that singleton babies are always more warmly dressed!
Disabled Access - When we were in Budapest we realised that our big red buggy would go anywhere a wheelchair could go - so over the past three months we have become very aware of the disabled wheelchair sign. In the US and Canada we could get anywhere (though it was very challenging in New York as there were only about three subway stations in Manhattan that we could use). But in France it is much harder and so we have had to carry the buggy up and down steps.
Routine and Bath time - We stuck to our 7am to 7pm routine and I think that is what made the boys so flexible with everything else. Sometimes it was like traveling with a couple of alarm clocks ... we would forget the time - but bang on 10.30am they would both start crying for their milk! DT loved doing a bath with them every night - he gets in the bath and does a boy at a time with me changing them in a production line. We took some plastic fishes with us for bath-time - they loved it.