Maybe you could explain a little more of your reasoning to do them together? How old are they? Do they have the same group of friends? Do you just want it family? Are you moving? Just based on what you've written I don't think it's okay to do them together, especially not in December when it's so close to the other child's actual birthday and Christmas, that seems the weirdest. If anything I could see you wanting to do both in July so the other daughter doesn't get overshadowed by the holidays.
My kids birthdays are one week apart exactly and I give them separate parties. I know that is odd to some, but I too feel like each child should feel celebrated and special on their day. I also like the way Tracy said that it's also an anniversary...it is and it makes me smile to relive that special day with them!
Even if you just do cake and punch on their birthday, I think it's better to have them separate. Plus they are in totally separate seasons so you could do so much more with separate themes, gift ideas, etc.