I'll be quite frank, and I know most others won't feel this way, but I do both extension courses, and they take up less time for me than my other subjects (all three sciences).
I don't even spend time at all on Ext. English except maybe a day or two before the exam, and the odd day or two other than that throughout the year (literally). Even for Advanced English, I probably do something for it a couple of times a week, and some weeks I don't even touch it. I did well in all my assessments for these courses so far by the way - I wouldn't be telling you this stuff if I did badly.
Maths...well, as Lumenoria said, it depends on your natural ability. Maths, for me, has always been my strongest point, which is reflected in my results, but I don't do much work for it either, not during the school term. I do the most work for Maths during the holidays, when I read ahead for topics we'll be doing at school, do all the past papers, do extra worksheets, etc. This is what you should be doing as well (regardless of which Maths course you do) as you go into Yr 11, and this essentially removes a burden off your workload. I'm not even up-to-date with all the exercises we're supposed to do in class, but my extra work pays off in the end and serves as the thing to... break my fall (into bad marks
lol). I am among the top Maths students in my grade at a good school, so I do think my advice is valid here.
In conclusion, take both extensions, like Lumenoria said. I can't guarantee that you'll be doing as little work as me (in fact, what I'm doing is not good - you should be doing more work than I am
), but hopefully, if you follow my advice for Maths, I think you should find that you won't even feel like you're doing two extensions.
BTW, don't do 13 units AND two extensions - that's insane! Don't do SOR. You would rather do Music and no extensions.