pub fn get_mut(&mut self) -> (&mut St1, &mut St2) {
        (&mut self.stream1, &mut self.stream2)
    }
Acquires a mutable reference to the underlying streams that this
combinator is pulling from.

Note that care must be taken to avoid tampering with the state of the
stream which may otherwise confuse this combinator.
futures-util/src/stream/select_with_strategy.rs