I accelerated in Methods and am completing further this year (weird, I know)
During VCE there is a minimum amount of units required to graduate, I can't remember exactly, but it'll be on the VCAA website. My guess it is was around 16? Eg. units 1+2 count as two towards the 16. Otherwise, an exception is required due to certain circumstances (medical or other reasons).
I know many people in my year level failed the methods 1 2 exam and were allowed to continue on to the 3 4, but i strongly discourage that, as it is just simply too difficult.
For further, they are changing the curriculum next year, so I am unsure about what will overlap. However, this year, the only overlap are in the modules. Matrices, trig and graphs and relations are all covered in methods to some degree, but all have other things you will have to learn within them.