I'm not sure if you HAVE to learn the 2unit Advanced Maths Course if you are doing Ext 2 Mathematics, BUT I'm pretty sure if you don't understand the 2u Advanced Course then you probably won't get Ext 1 and Ext 2 Maths, so it would be wise to learn it.
At my school, they try and make it easier for us, and they let the top class accelerate. So if you're planning to do Ext 1 maths and you're in our top class, they'll force you to accelerate by doing all of 2u in Year 11 and then all of the 3u course in Year 12 - less stress. If you're planning to do 4u maths (provided you are good enough) then they will let you accelerate both 2u and 3u in Year 11 and all you sit is Ext 2 Maths in Year 12 (unless you do bad in 3u in Year 11) - less stress for next year.
As for the unit count it goes like this:
- If you are doing 2u maths - Mathematics (Adv) counts for 2 units
- If you are doing 3u maths - Mathematics (Adv) = 2 units, and Maths Ext. 1 = 1 unit, therefore 2+1 = 3u
- If you are doing 4u maths - Mathematics (Adv) does not count and you cannot sit the exam as its the same time as the Ext 2 Exam, Maths Ext 1 = 2 units, Maths Ext 2 = 2 units, therefore 2+2 = 4u.