Originally Posted by thewind32

And speaking of upgrading skills, suppose I upgrade a knight with 2 copies of the same girl in order to level up her skill. Is there any difference in the RNG % between upgrading once using both copies together, or doing them separately? (Apart from gold costs due to leveling up from the first one).

If you use multiple skill up materials at once, they will act like they get fed one at a time. You don't get an upgraded chance at one skill level up. You can test this yourself using 2 star girls from normal gacha.