K.C.
I think you can find a way to combine the themes. Send princess invitations to your 8 year old's friends and whatever theme the older one chooses to her friends. If you can get them to agree on a color scheme, you can do a lot of solid color paper goods, instead of themed ones. You can even let them each choose one color, if they go together, and have some of each. What does your older one want? Are you doing the party at home or at a venue?