#[derive(Debug, Default)]
struct Foo { a: String, b: String }

impl Foo {
   fn get_mut_a_b(&mut self) -> (&mut String, &mut String) {
      (&mut self.a, &mut self.b)
   }
}

fn main() {
    let mut foo = Foo::default();

    {
        let (a, b) = foo.get_mut_a_b();

        a.push_str("Hello");
        b.insert_str(0, "World");
    }

    println!("{:#?}", foo);
}