I think this is one time when you'll really need La Leche League. I tandem nurse, but my other nursling is more than two! It's not very difficult for me, but Linda can understand "You need to wait a minute." I have talked to several women online who successfully nursed twins, so I know it can be done, but it's even more unusual than nursing at all, so I think you'll be well served to have a support system in place before your kids are born, especially since twins quite often come early.
I wish I could give you more specific advice, believe me. It might not be your cup of tea parenting-wise, but I really recommend checking out the fora at
MotheringDotCommune for breastfeeding help, I know of at least one lactation consultant who's making a point of visiting their breastfeeing forum. LaLecheLeague.org will help you find LLL where you are, and they have a page of info on feeding twins:
http://www.lalecheleague.org/NB/NBmultiples.html & also a
"Breastfeeding Help Form" that may be of use.
Just speaking generally, I have found that giving the elder sibling things to do to help out with the new baby usually helps. I don't know how old your son is, but Linda was about 21 months old when Esther was born, and even at that age could do things like bring me diapers or a blanket or something. (It has also been my experience that my kids are capable of doing a lot more than I think they are!)