Both of my extension 1 english and maths classes had people who were getting not-so-great marks in year 10. Some of them dropped out pretty quickly but others stayed. The doing extension english will definitely (directly) help with advanced, while maths doesn't help as much, apart from extending yourself so advanced seems easier.
Also, think about if you can handle 14 units for at least a term. I didn't have much of an interest in extension english and it is now my favourite subject. I held onto it going into term 2 and dropped music which was my favourite subject in year 10 (it just didn't end up being what I wanted it to be). I had the ability to drop a full 2 unit course because I had both extensions and had 14 units.
Basically, just go for it, you'll have the ability to drop them and they'll assist in their 2-unit versions.
