So after doing a long 4 hour crafting session, I'd have to say that the Double Synthesis Skill Chance does not appear to be working whatsoever.
Trying to get one of my character's Bonecraft skill from 44 to 60.
The Horns went pretty well, skillup every synthesis until I was 2 levels from capping and even then it went pretty well.
Scorpion Arrowheads? Went nice for 2 levels then it slowed down something terrible. Managed to scrape out until 1 level under the cap.
Then.... Scorpion Rings. First 2-3 levels went fine, and then once I hit 55..
........nothing. 10, 20 successes in a row, no skill. None. Finally got a 0.1 after about 25 successes.
Started the session with 14 stacks (168) of Scorpion Shells at Lv52.
Used all but 4 stacks of them on Scorpion Rings. Only got to 56 (that's a whole 4.0).
Then I got some Ram Leather, and used the rest of the Scorpion Shells on Scorpion Masks. Got to 58.
Then, family member gave me 4 stacks of Ladybug Wings. That only got me to 59.6.
So you're trying to tell me that ~200 synthesis attempts should only get me 7.0? And that's supposed to be double the Synthesis Skillups?
Then I did some Tiger Leather.
Started on 53, Lightsday. Had 12 stacks (144). Barely got to Level 59 with 13 left. (131 used).
All of the above was done while eating Kitron Macarons.
Instead of applying a bandaid like limited-time events to help us gain Synthesis Skills, why not adjust some of this stuff so that we can actually skill up in a reasonable fashion, instead?
200+ Synthesis attempts for a measly 7 levels is not reasonable, especially not if that is supposed to be "double" the rate (that's implying that 200 attempts would normally have only given 3.5 levels).