I've gone to joint birthday parties for siblings, but I've never been invited to one for classmates. (rookiemoms.com)
My brother's birthday is only a couple weeks before mine... so, we had a couple joint birthday parties in our time. (have-clothes-will-travel.com)
I actually (strategically, as an 8 year old), managed to do joint birthday parties with my friend Jeanne who's birthday was September 27. (sprint2thetable.com)