I am a year 10 math accelerated student at the moment and will be doing hsc for advanced and ext 1 Math next year in 2024. I am planning to take extension 2 in year 12. If I decided to do extension 2 in year 12 but perform very poorly in the hsc. Would they take my result from ext 1 & 2, or would they take my advance and ext 1 result?
You should never be thinking in terms of results, you should be thinking in terms of content acquisition and suitability for future studies.
If you have the option to take EX2 AFTER finishing Adv+EX1, I would STRONGLY recommend that you do it. It will give you time to consolidate your understanding of the EX1 content, before moving on to the EX2 topics.
Every EX2 topic is essentially a continuation of Adv+EX1 topics: Complex Numbers is a continuation of algebra/trig/polynomials, proof is a continuation of proof, Vectors is a continuation of algebra/trig/vectors, integrations and mechanics are continuations of algebra/calculus.
If you are planning to attend a high-ranking university to study math/stat/engineering/finance, having taken EX2 will be very convenient, and having taken it in year 12 will mean that the content is still fresh in your mind as you are heading into the university math/stat/engineering/finance courses (many of which will either exactly use EX1/EX2 content, or will move ahead to advanced variations of the EX1/EX2 content).