i know people who've started off with this combo and dropped something out of sheer stress. the one that's usually first to go is 4unit english, which i think is rather a pity ]:
well, i can't really say much about it, because i personally don't take that combo and i absolutely despise maths (while absolutely loving english), so i would find that combination an absolute horror.
so yeaap. but do keep in mind that both extension twos require a LOT of work and dedication and you may find that you can't keep up with one of them. i think it would be best to try it, and then see how you go. but as you say, it would probably be easier because you've narrowed down your area of subjects to a few.