Board Thread:Theories and Speculation/@comment-30168130-20161223235115/@comment-25122992-20161224112148

In fact the thing is that the combined jutsu of Naruto/Sasuke wasn't an actual "Scorch Release", it was just a combination of Wind and Fire (Blaze) Release, the same way as Typhoon Water Vortex Technique was just a combination of Wind and Water and not a combined nature. It was Minato's idea to call the jutsu that way, but it's not entirely correct. The combined natures can be created only by the KKG users, while neither Naruto nor Sasuke have a KKG for Scorch. The only example of the genuine Scorch Release was Pakura's use of it, since she was known to possess this KKG.

Per Yamato's explanation, it's not enough to learn 2 elements, you must have an inborn affinity for both (and all the normal shinobi have only 1 prime affinity) and thus an ability to use them simultaneously. That's why KKG directly means "a bloodline limit", since only certain bloodlines have such an ability. Regarding Mei, it just seems she was born as a descendant of the 2 different bloodlines with 2 different KKG (Boil and Lava).

About Oonoki and Muu, their Kekkei Tota is placed above regular KKG, but its principles weren't explained properly. It means "a bloodline selection", which is kinda difficult to unterstand. There was a quite plausable theory that KKT could appear through the celective breeding of the two or more bloodlines and that's why it's so rare. Also, I must note that we don't actually know if Muu and Oonoki aren't related, so we can't state it as a fact.