How many threads do I need to earn or upgrade a trade?

So the trades go like I. 5 basic threads for trade -> II. 10 specialty threads for trade -> III. 20 specialty threads for mastery of trade. The last 20 have to be specialty threads too, right, not just 'threads for trade'? How does it work when you have 2 specialties? You'd need only 10 threads for mastering your trade in step III then, right (as you already had 25 from step I + 2x step II)? But do you choose 1 specialty to master if you have 2, or do you master the trade in general..?