I want to be good with C++ as that's whats used in my industry the most.
I also want to be good with Rust as that's where the industry is heading.
Is this mentally possible? Or am I going to get overwhelmed?
It is entirely possible. What you’ll find is that your C++ code will become more and more like Rust in how it manages ownership of things. The better you get at this the fewer bugs your C++ code will have.
If you're using both languages regularly, yes. Personally I find that I can't retain things that I don't use, especially if it's a huge language that I've barely learned in the first place by practicing in my own time.