How does this work?

You can't be both without the aid of Hircine's ring.

If you have lycanthropy and start the Vampire side to the campaign you will have the option to Forsake your Lycanthropy for Vampirism. When you take the option you will lose your transformation for the Vampire lord form. If you choose that you do not like your new form you can go back the Companions and get Lycanthropy back, i dont know how many times you can change back and forth so i would just think on it before you do.

Hope this helps,
Thanks

Already tried that. Won't work. atleast, not on my xbox version of the game.