i think the issue of acceleration depends on the subjects themselves, certain subjects are easier than others and some subjects are simply not suited for acceleration.
For example english or history is generally not recommended as an accelerated subject as it takes time to build up the depth and sophistication of essay writing skills. There are some conceptual areas that one year makes a big difference in the understanding of.
However a subject like business studies (which i accelerated) was a complete breeze to accelerate because it required almost no prior knowledge (i never did commerce) and is very much content based.
Maths, if one has the capacity is quite do-able, and i know plenty of people who have accelerated it, but the level of difficulty rests on the level of ability and the number of maths units accelerated (ie. 2/3/4).
that said, the original poster might not be talking about acceleration? I suppose completing HSC over 2 years may refer to something like what Daniel Swain did.
which was do 3/4 subjects last year (Drama, M History, 4U English i believe) and do 2 subjects this year (Cosmology, Society and Culture).
that's doable, but again- all about individual strengths and ability.