I know two people who took both ext 2, and they ended up dropping one of them because it was too much effort. You can always just pick up both and try them out, because english e2 is a little different and shouldn't take too much time in the first term, but probably don't expect to be able to keep them both. Also, don't feel obligated to take mx2 even though it scaled so well, because it is a big time commitment. Personally, I was ranked top 5 in yr 11 mx1 at a top 10 school, but dropped mx2 after 5 weeks because so many people had planned on doing it beforehand and were VERY ahead in terms of content - it didn't feel like a fair game.
I wouldn't recommend sacking chem just yet, because you'll have to stick with it the whole HSC year, and who knows if you'll end up doing better in it or worse in another subject? You can drop eco now though, because otherwise it'll really affect your results next term (which are of course not that important, but still matter, especially if you accidentally flunk a couple of exams due to the workload).