I am a mom of twins, now 11, so I have been thinking of a few tips:
Did you say whether you were thinking of a 3,4, or 7 day? - just a heads up, twins will require lots of formula (if you don't nurse), diapers and baby food (depending on their birthday - remember many twins are early)....that's a lot of luggage
another thought - many of the doorways are fairly narrow and you may want a front/back stroller rather than a sidebyside
My personal experience was that at 4-5 months I was just beginning to handle the workload (it's the "maintenance" that is rough) and be functional (enough sleep to think straight)... mealtimes were an adventure since they both weren't able to entertain themselves at the same time.
I know all kids are different - some babies are great at self soothing and some aren't....IMHO - I can't imagine going on the cruise with twins this young
hopefully others who have done so will be better able to advise (I just think one baby is a snap compared to two. DIS parents who describe their cruise experience who just brought one...well, as they say, your experience may vary)
I hope you stay pregnant a good long time (even though eventually you will wish otherwise) and have lots of help!
Barb