If you're going to have their birthdays together, I would not recommend segregating it. Guests who are friends of both, at this young age, may have a difficult time, feeling disloyal, if they attend one side of the room and not the other. If you want to combine elements, maybe Abby Cadabby could meet them at the door (like a poster or something), and you can have Hannah Montana music in the background.
For the invitations, I see Abby Cadabby on front, saying something like "Presto! A surprise!" and inside, Hannah Montana says "join us to celebrate the birthdays of TWO wonderful girls!" and give the info. You'd have to print these yourself, but there's so much clip art and photos online, it shouldn't be a problem.
at 9:11 PM on Oct. 15, 2008