My twin boys are five. I have their first year book, and it's together. For each month, I made a page of each of them, and then used the We-R-Memory Keepers for all the extra photos I wanted to put in. The book also includes special events like Christmas and their first haircut.
I scrap them the way I treat them. They are individuals but we are still one family. And if they ever want to separate the book, it'll be easy. If they don't, I won't have duplicates of everything. I'll just have a specific page of each child.