Why did Leah first say "Hashem saw", and afterwards "Hashem heard"?
Hashem saw that she was Senu'ah and she conceived the first night, before she prayed. Shimon was conceived after she prayed. (PF)
Why did Leah say "Gam Es Zeh"?
Tosfos ha'Shalem (2): A twin sister was born with Reuven and with Shimon.