Hi there,
I'm not 100% sure about this, but looking at the product screen shots it looks like you have to add it twice with 2 different names. Check out this screenshot from the product page in the SEBLOD store:
http://www.seblod.com/images/medias/165/search_ordering_01.jpg
See how they have seb_item_price_eur and seb_item_price_eur_2 for the ASC and DESC options? Maybe you have to try something like that?