I bought Elite Collector's + Statue on Fig, then I bought Additional Collector's Edition Box on Obsidian site. Now I see, that I can generate only one key -Pillars of Eternity II: Deadfire - Ultimate Fig Edition. What about the additional box? How it works?

You pledged for an additional box, which does not include an additional digital key. You could open the second box and use the key inside of it, or go add on a digital key and use that.

Thanks for reply. The thing is I add collector's edition box which include digital key. 

 

http://eternity.obsidian.net/backer/addons

 

An additional, sealed Collector's Edition Box for Pillars of Eternity II: Deadfire, including all the box contents.

 

Let's say you're right, any idea what key gonna be inside the box? Steam or GOG?