Hi V.! I am godmother to my sister's girl/boy twins. For their first birthday we did a pink and blue party. Guests were asked to wear pink or blue and most of the food and drink was pink or blue (pink lemonade, blue Hawaiian punch, red and blue M&Ms, cotton candy, strawberries, watermelon, blueberries and so on). We did use the 1st birthday boy and girl themed decorations from the party store. Whatever you do have a great birthday with your twins!
P.S. This thought just came to me, but may not be up your alley...my sister decorated her twins room with Noah's Ark decor, the theme being two by two. Don't know if that would work for you, just a last minute thought.