Assigning instance field inside method Look at the method setupCamera. I use it to reduce the size of the create method. As you can see, inside this method I instantiate camera and viewport, that are fields. 
Is there any problem to do that, or exists  there any surer way to do that?

public class HudCameraSample extends ApplicationAdapter {

    private static final float SCREEN_WIDTH = 1280;
    private static final float SCREEN_HEIGHT = 720;
    private static final float SCENE_WIDTH = 12.8f;
    private static final float SCENE_HEIGHT = 7.2f;
    private static final float WORLD_TO_SCREEN = SCENE_HEIGHT / SCREEN_HEIGHT;

    private OrthographicCamera camera, hudCamera;
    private Viewport viewport, hudViewport;
    private SpriteBatch batch;

    @Override
    public void create() {
        setupCamera(camera, viewport, SCENE_WIDTH, SCENE_HEIGHT);
        setupCamera(hudCamera, hudViewport, SCREEN_WIDTH, SCREEN_HEIGHT);
    }

    private void setupCamera(Camera camera, Viewport viewport, float width, float height) {
        camera = new OrthographicCamera();
        viewport = new FillViewport(width, height, camera);
        camera.update();
    }

    @Override
    public void render() {

    }
}
Is there any problem to do that, or exists there any surer way to do that?


Yes, it's completely broken as in setupCamera you're assigning local variables and the fields remain untouched. There are no var parameters in Java. You'd actually need something allowing to return both camera and viewport. This is doable somehow, but too verbose for the little code you're trying to minimize.