I would really just go with the same theme for both. Party stuff is expensive and if you're doing two completely different themes, that's going to get up there. You mentioned seeing a few pink pirate items. Just get her a couple of those. Get her a little pirate costume and she can still wear the crown too! She'd be adorable. :) Besides, she's too little to really care. This party is for you, mamma. :)
Two of my children were born so close together (my 8 yo & my 4 yo) that we always celebrate on the same day. They decide together the theme they want (if we go with one) and then we go from there. So for instance, last year one wanted princess and the other wanted dragons (usually they agree better but it still worked out). But I didn't have the budget to buy decorations anyway. So I made one huge cake. I put the kids Lincoln Log castle on one end, stuck a little princess doll in the tower and then put knights and a dragon on the drawbridge & lawn. I even had gummy fish in the moat! The kids thought it was awesome. All the kids came in costume (if they felt like it) so we had princesses, a few knights and even a dino running around. :)
A friend of mine is in a similar situation (1 girl, 1 boy like you) but she just does a general theme. One year it was bugs, one year it was cowboys/girls (the cake was a huge saddle, it was awesome), one year it was construction, etc. You get the idea. Both children always really enjoy their parties.
My brother & sister were born on the same day (1 year apart) and we rarely had themes other than the cakes. My mom made a separate cake for both of them, and decorated it however they wanted. It wasn't until they hit their adolescent years that they wanted separate parties and that was only because if my mom did do a theme, she had a bad habit of letting my sister have all the say so my brother felt ignored & left out. As long as you make an effort to include both children and try to listen to their (reasonable) requests, you should be good to go. :)
In my house, also, there is a three generation birthday every Thanksgiving. I was almost born on my dad's bday (my mom went into labor on it), and my youngest son was born on mine. :D So we always celebrated together. My dad gets his pumpkin pie with a candle stuck in the middle, it varies for me and a small cake is always made for my son. :) It's a big thing and I've always enjoyed it (except for the years no one made an effort for me as a child and I didn't even get a pie).