As a mother of twins (now about to turn 5), I appreciate your ambition and they seem pretty seasoned already. A couple of years ago, I booked a cruise and posted on here asking for tips. Some folks were so rotten that I wound up cancelling it. I was nervous about it to begin with and some of the comments just made me not want to go.
That being said, we have traveled plenty of other places with our little beasts. I second the umbrella stroller recommendation and suggest you see if you can figure out a way to carry the umbrella strollers when necessary (i.e. some sort of sling, backpack or whatnot) for those times you are faced with stairs and have to carry the stroller and the small humans down (or at least hold their hands).
Have a wonderful time!