I'd just do one subject (i.e. old method) unless you're pretty sure you'll like Corps. Personally, I didn't find corps very interesting. A lot of people find it boring, especially those with no interest in commercial stuff. I majored in finance but still found corps comparatively dull. It's not the hardest unit, but (at least at undergrad level) it is very crammed, which is why I'd be hesitant about a 2 in 1. Quite a bit of Corps Act interpretation is involved. Not all that many cases, except when you get to director's duties, and then there's a tonne of them. DD is one of the more interesting areas, but still kinda dry. Even the lecturers will sometimes complain about the dryness of corps. Having said all this, some still find it interesting, mainly those with commercial/business interests and/or those clerking in the big firms.
It won't really matter that you haven't done corps before the clerkships. It'd look good if you pulled a D/HD in corps (assuming they noticed), but ultimately they basically only care about your law WAM.