they are saying that they are the strongest duo in the world at that moment (or maybe ever) xd jk they mean that nothing is impossible when those two are used in tandem and that's why kabuto used them to get last beasts (little did he know itachi pwns even in the afterlife)
Probably not, it has been hinted by Shikaku that Itachi is able to control multiple shinobis at the same time. Tsukuyomi inflicts pain, it's not meant to brainwash people.