M.D.
I say go for it. My kids have birthdays in July & August, they are 25 months apart. They have always had joint birthday parties. They love it because they get to have a bigger party because they share. Neither child has ever known a party of their own and they are fine with it. We celebrate with a special dinner and presents for them on their actual birthday and one big party. No guests have ever complained about bringing two presents, just think, if you had two parties in a two month span the guests would likely by both presents at the same time anyway. If a joint party is what the kids want, go for it, it's all about them anyway. :)