i would do them individually - when they are older, it is easier to give them their own books than to work out who gets what. you could have photos or pages that cross into both books because both kids are in them - and then if you are journalling, you can write the messages more about the child that the book is about (so, when DS was born, you could have the comments about him meeting his big sister - and in hers, about how she reacted, how she helped etc). it might come up a little more exxy to do it that way when it comes to printing, but i would do it so that there are no issues later. if you're doing "events" you could print two copies of the exact same book for them to have later.