Linking symbols?

HI all,
Not exactly sure what I want to do, but I think I want to link 2 symbols that are used for a mouseover button. Why? Because when I create a shape (white rectangle) in order to create an ‘erasing’ effect and appearing effect for some text (on a bitmap), the opacity (alpha) is 50%, like the original shape’s alpha. However, I don’t want this! But I cannot find out how to change the alpha of my new shape alone, separated from the original shape. So I thought just to create a new symbol and link them, so I can change the alpha separately and easily…

Thanks for considering!