Hey there! I'm currently in Year 12 and I'm doing HEX and 4U English, so I can't tell you what my ATAR/HSC marks are (yet), but I do have some personal thoughts about it that may be helpful 😃
I think if you have a passion for subjects that require lots of reading and writing, then both the extensions definitely go well together and will utilise your skills to read wide sources and craft varied pieces of writing. However, I would recommend really planning out your time if you have these subjects.
I've already submitted my essay for History Extension, and am now purely focused on completing my Extension 2. I do regret not spending a more even amount of time on both the works a bit earlier, so my advice would be to not get tunnel vision.
Although it certainly has a fair share of challenges, I think if you like history and English it won't be as much of a chore as some other subjects, especially if you pick topic/s you like! I also enjoy the different writing styles, as when I wanted to be more objective and factual, I would work on my HEX essay, but when I felt more creative I go straight to my Extension 2. Additionally, if you're thinking going to uni, then HEX is a really good course to look at, especially with regards to learning how to research, correctly cite and use sources, and write in a more academic register.
Give it a go is my main advice if you're on the fence about it 🙂