Now and for ever you give them each a present suited later on to each child's preferences, a card each and if you bake a birthday cake, you bake two.
After all, you would not give two children born a year and two weeks apart joint gifts, would you?
If the twins themselves object to having two of all this at any time, you can obviously change what you do.
This year, I suggest you buy one twin a copy of "Winnie-the-Pooh" and the other a copy of "The House at Pooh Corner" in book form, I mean. I still, at the age of 71 have my copies and read them regularly with much the same delight as I did as a child.
Or you could, this year only, buy something big and expensive that their parents want for them as a joint present, if your means allow this kind of generosity.