That is correct, if you accelerate Mathematics Advanced and subsequently take Mathematics Extension 2, your result in Mathematics Advanced will not count towards your ATAR, with Mathematics Extension 1 and Mathematics Extension 2 each contributing 2 units towards your ATAR.
It is therefore important to carefully consider whether you should take Mathematics Extension 2, especially if your HSC result in Mathematics Advanced ends up being of a high standard. Being a more difficult subject by nature, performance at the same level or exceeding the level at which you perform in Mathematics Advanced is likely to be more difficult to achieve in Mathematics Extension 2.
I hope this helps!