I've watched their use of YaYa change over time. At first, YaYa meant both me and other. So Maddie would say, "YaYa ball," both if she were holding a ball or if she were commenting on the fact that Riley had a ball. It seems like it could have been confusing, but it was always clear from context who was being referred to, and I loved that they had the same name for each other. In fact, I think that their calling each other the same name was in part connected to their not totally understanding that they are two separate people.
As they got a bit older, the use of YaYa changed. For a few months now, YaYa has only meant other. Riley will say, "I have snack," or "Riley want it" when referring to himself, but "YaYa listen music," or "YaYa wear pink shirt" when talking about Maddie.
Today, things changed again. I overheard this conversation:
M [handing Riley a toy]: Here you go, YaYa.
R: I no YaYa! I RILEY.
All right then, big man, Riley you are. And you're growing up.