The cribs I've seen are port-a-crib size so it would depend on how old they are and the cabin catagory. You want a good night sleep yourself so be sure they are also comfortable and not waking each other up.
Teresa and Larry
#36-Carnival Splendor 9/16/12
7 night Mexican Riviera
#37-Allure of the Seas 11/11/12
7 night Eastern Caribbean
I think you should talk to the cruise line directly for details. I know one of the times we cruised with my son, I told them we needed a cabin for three people (myself, husband and older daughter) and a crib. WEll, the crib arrived but there was no where to fit it, even though this was a nice, outside verandah cabin. So I think that two cribs will be really hard to fit. Sometimes you get small, hotel style cribs in which only one baby could fit. Often, though, you get the portable playpens in which two babies could fit. Once you get some answers from calling the cruise line directly, I'd call back a second time to make sure you get the same answers from someone different -- that way you'll know if it's for sure or just someone assuring you they'll fit (which I was) when in fact one or two cribs might not fit!
Please email me directly: firstname.lastname@example.org once you get back from your cruise. I'm very curious to see how it hopefully all worked out well!
Luisa Frey Gaynor
Family Cruise Editor