Its common knowledge that if you do both extension subjects, your mark for one of the extension subjects will drop. There's a minority of people that are able to take on the workload of both extension subjects and they have a love and passion for doing both subjects as well as the willpower. I think its a little early to consider your year 12 subjects because you'll understand what you want to do when you reach there.
I did a mix of non math subjects and math/science and I've found that when you start to enjoy one side more, you typically start to lose interest/drop off in other subjects because the learning approaches are conventionally opposite. For 3u math, I doubt you would get anywhere with memorisation whereas its a staple in doing HSIE subjects such as legal or economics. That's just me and theres a possibility thats just how I feel, there might be other people who personally enjoy the mix of different types of subject but I think its more efficient or effective if you take subjects that complement each other so your workload is indirectly lesser. For example, if you take physics and maths, you'll find some direct synergy in topics however the best part is that the learning styles are similar in that its mostly conceptual. Thats just how I perceive the subjects at this point and its a common strategy to go for maths, chem and physics.
3U english and maths require significant amount of dedication but its doable if you plan it out correctly and optimise your time. Personally the workload is too much but there are many people who did it in year 11 to get a feel for what subject would work better for them in year 12 such as mx2 and english extension 2.
You're going to have a large workload in year 11 with your combination and you will do your economics HSC that year as well. If you know you're capable then I don't see anything wrong. Make sure you study a bit of the subjects prior to selecting to them to get a feel for the content, thats what I did and I was able to rule out some subjects I would have regretted selecting. Once again, this is all my opinion and its purely based off my experience in year 11 so take it with a grain of salt. Asking your tutor/teacher or other people such as older siblings or friends might help you as they would know your strengths and weaknesses.
Regardless, all the best.
