My 3 and 4 year old are a year and nine days apart. To spice it up, they are a girl and a boy
We have a combined party for them while they are young. I never really gave them the option of choosing their own theme individually. Their options always involve something with two main characters. We have done Dora and Boots. VeggieTales has Bob and Larry. This year we are going to do BluesClues as my daughter loves Blue but my son loves Joe or Steve if its a rerun. This is the last year we will worry about it as I won't combine their birthdays once they get into school.
For my son's first, we got little cakes for both of them. She had Barbie and he had teddy bears. We split the table in half on the decorations and let them each have their cakes. Then I bought a very generic looking cake for the rest of the guests. We also only have close friends and family at their parties at this age. I take cupcakes with their theme to the Day School and they celebrate there with their friends.
Good luck with the planning and I HTH