You know this is a question that has been bothering me as well! I mean, I don't know, I was taking it as a given that both of them will somehow know about the others' kids, but now I am not so sure.
When I think about it, Yash probably did not know that Aarti had a son, until Pratik overheard his mom and went and told Yash about the ridiculous shart she was keeping. Had it not been for that incident, Yash still wouldn't have known because the only information he's gotten about Aarti is from his parents, and they've been extremely to the point, not divulging any details.
The same goes for Aarti, while Yash has said yes to the punar vivah, Aarti has yet to agree. Her MIL also just mentioned Yash, and she got to know a little bit about his character through the letter she wrote her, but other than that Aarti is completely clueless about him and his family. After the whole misunderstanding and condition fiasco about Ansh, Aarti refused to listen to anything about Yash, and so I really don't think, at this point, that she knows that Yash has two daughters, and he's looking for a mother for them, not a wife for himself. I really hope though that they clear this soon enough, and if Aarti doesn't know, then she needs to be told, because she needs to wrap her mind around the fact that all of a sudden she'll be going from being a mother of one to being a mother of three!