I thought I had seen this discussed here, but now I can't find the thread.
I have 11 week old b/g twins. They're starting to smile at and interact with their big sisters, but they don't really notice each other. I figure they're just such a part of each other that's always there, that maybe they don't notice or realize that the other is independent of them. I don't have anything to back that up, though. :) When did your twins start to notice each other?