My 2 oldest children are 3 years and 2 days apart. We had combined birthday parties until last year when they requested separate ones (boy and girl - so very understandable). We have 2 parties on 2 different days - 1 on Saturday afternoon, 1 on Sunday afternoon. I don't know too many people who have their party on their actual birthday so it's not a big deal... we've never had or gone to a mid-week party!
Since your girls are closer in age, a combined party would work pretty well. You don't need organized games for them yet... my oldest just turned 7 and this is the 1st year we had organized games at his party. The kids have alot of fun just running around playing. You can have 2 sets of games... just enlist the help of another parent or your husband to run the games for 1 set of kids. Most parents of 2-4 year olds will stay at the party because (at least around here) drop-off parties don't start until kindergarten - age 5.
As for party theme and such... have 2 tables set up, one decorated with Hannah Montana and the other with Mickey. Also decorate the tables to keep the gifts on to make it easy to keep them separated.
Another option is not have it at your house, although the cost skyrockets at that time (our parties rarely go more than $40-$50 for 10-15 kids). A place like Monkey Joe's would be perfect... all the kids will love it and be entertained by running around and jumping and you won't have to worry about games.
Personally, though... if they request separate parties you should give them separate parties. That makes it more special to them... not too many kids prefer to share the spotlight of a birthday party with a sibling -- even twins and triplets I know get separate parties!