I have not seen any joint birthday invitations. But, you could use Publisher on your computer and create one of your own. One of my friends writes a cute poem about her children every year at Christmas and puts it in her cards. You could do something similar for your daughters. My son's girlfriend's dad used to create a birthday invitation on Publisher for her every year. It is one of her fond memories of her dad.
Our best birthday parties were when our boys were preschoolers. We would get permission to use the church gazebo and playground for the fall birthday and the church fellowship hall and gym for the January birthday. We would mail out birthday invitations instructing parents to bring their child's favorite riding toy. We would bring a few extra riding toys for anyone who forgot. We had pizza, cake, ice cream, and a cooler full of juice boxes and little bottles of water. One year we even had a magician at both boys parties. Fun, easy, and cheap birthday party.