I would NEVER have a combined party. The whole purpose of a party is to make the birthday girl feel special. This is HER day, and I don't feel she should need to share it with her sister.
The rest of the year our kids are taught that the world does not revolve around them, they need to wait their turn, and they must share, etc. Their birthday is the one time a year that THEY are the center of attention, are "queen for a day".
If kids/babies are very little and don't know the difference, I guess it's not as big a deal. Otherwise, schedule the two parties when it's good for you, and the family members who can make it will come. Don't fret about the guests, it's all about your daughters.
PS-I think I'd feel the same way about twins. Just because you are born on the same day as your sibling, you shouldn't get jipped on sharing the attention....unless you WANTED to share the party.