Board Thread:Theories and Speculation/@comment-27295021-20180308000450/@comment-4974066-20180308005149

Merging 2 natures together is a Kekkei Genkai, yes. As stated, they cannot be learned except for in Rasa's case. It's also possible for 1 person to have multiple Kekkei Genkai. If a person with no Kekkei Genkai were to use 2 natures together, the result would just be some jutsu that has properties of both natures. An example would be Darui during the Kage Summit Meeting. When he attacked Sasuke, he used a jutsu called 'Lightning Release: Wave of Inspiration', which the 4th Databook describes as a fusion technique (融合術, Yūgōjutsu) that uses both Water and Lightning together without "merging" them. But Darui also has a Kekkei Genkai where he does in fact merge Water and Lightning, called Storm Release. Also merging 3 is a Kekkei Tōta, and theres only been one example so far of this ability called Dust Release which merges Fire, Earth and Wind.

As for Yin and Yang release, Yin is representative of the imagination and spiritual energies and Yang deals with the physical energies and governs vitality. Yin and Yang are outside the 5 basic natures and have their own set of rules. it is explained in the series that Genjutsu falls under the category of Yin Release. This means that all genjutsu are Yin but not all Yin are genjutsu. The Nara clan's shadow jutsu are also Yin Release. Yang Release hasnt been explored to the same degree that Yin has, to my knowledge. However, the Akimichi clan's Multi-Size jutsu falls under Yang. Many people in the Naruto community also believe that healing/medical jutsu also falls under Yang Release, but the series doesnt confirm this.