I have a boy and girl twin siblings.
At birth we got half and half, some people gave one card for both and the blue pink thing aswell, while others gave 2 cards and 2 different pressies.

Now they are 7 so recieve individual items, although they have only one b/day party, most people give them seperate items and alot of their friends only give one present.
They are old enough to understand all of it.

It depends on what you want to give, I dont see a problem with either as long as your acknowledging them both, it shouldnt matter.