Hey I don't do the jd (I'm at USYD doing combined law) but it appears from the unsw website that accelerating the jd to complete in 2 years is just doing subjects in summer school- rather than overloading during normal semester. As such I don't think the workload would be much different from doing the jd in 3 years, you just don't get the big long break in the middle. From my experiences at USYD however you should probably check a few things-
1) at USYD you can accelerate your law degree by doing law subjects in summer school
However there is no hecs help- you have to pay full fee for those subjects- as such it is expensive.
2) at USYD summer school subjects are limited- this wouldnt prove a problem for compulsory subjects but you may find there are a more limited selection of electives offered at summer school.
Obviously these are things I have noticed at USYD- but they may or may not apply to unsw, so I would check

as for any kind of accelerating a course that does involve overloading during semester (eg doing 5 subjects instead of 4)- most people would view that as suicide for law (although unfortunately many people I know seem to have the opinion that you need to be doing only 3 subjects a semester to be competitive, which I think is silly, but anyway).