Just used two of these to make a Gem Stone Shadow Shield II but as I haven't seen anyone post how it works figured I'd let people know since I had to file a ticket to figure it out.
But to use Gemstone Shadow Mix Recipe you do need two of these items in your inventory which are both consumed along with the 3 Shadow Gemstone pieces at +7 you select in the pop-up box.
Also if you attempt to use Gemstone Shadow Mix Recipe with only 1 in your inventory then the interface will open but you will be unable to complete the exchange and the one in your inventory is consumed!
Hopefully the item's description can be updated to say it needs two since that is missing