When making a birthday cake for twins, personalise it for each twin. You can do this making two separate cakes or by making one cake, which showcases the interests of each twin. Doing this is important so each can feel her birthday is special. Avoid making twins feel like their whole identity is being a twin. Making a birthday cake for twins can be very inexpensive, and can take very little time.
Make two cakes, which are similar but still unique. For example, you can make two cakes that are decorated the same, but are different flavours. This way each twin can have his favourite cake. You could also decorate the cakes with different colours. You could also make cakes with objects that go together. For example, one cake could look like a bat while the other cake looks like a ball.
You can make two identical cakes. Each cake will have one name on it so each twin still gets her own birthday cake, but the cakes will otherwise be identical. This idea is better when the twins are young, much like dressing twins the same.
Bake two cakes and decorate each cake with each twin's personal interest in mind. For example, if one twin is a cheerleader and the other is in the band, you can decorate one cake with a cheerleader theme, and the other cake with a band theme.
Bake one cake, but decorate it to fit both individuals. For example, if one twin plays basketball and the other twin collects Micky Mouse, you can make a cake with Mickey Mouse shooting a basket in frosting. It can be a bit detailed, but it would still take less time than decorating two cakes. This idea is helpful when you don't have the time or resources to bake and decorate two cakes.
Let the twins each decorate their own cakes. This is particularly fun when the twins celebrating are kids. Bake two cakes, and then let each twin decorate one cake in the colour and design he wants.