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the elemental techniques for naruto is actually explained in the manga how that possesing at least two elements is signs of that of a kekkei genkai blood trait and it seems that most people that possess a kekkei genkai trait is capable of using two elemental techniques such as sasuke fire and lightning, itachi's fire and water, the third hokage fire and earth, haku water and wind makes ice, and, yamato/hashirama water and earth makes wood. But I dont know how kakashi fits into this but maybe because of the sharingan he can posses more than one the whole everybody can gain two elements on their own is kinda stuff that the anime makes on their own. the people here in the guild that uses elemental techniques most of them have bloodlines so accessing two elements for them is there for possible.

If only he asked Yamato to take a card, because I don't know how it works for those who have affinity to more than one element...

Yukki focuses her chakra, then the card cut in half and incinerated. Yay!

Yeah thats correct but we limit the use of the chakra elements for those who either match their level or have a kekkei genkai blood limit for people at genin level can use one element.but some kekkei genkai's allow two people can naturally gain another element when they reached jonin rank or so and three when reached kage rank but as what jiraiya actually stated in his battle with pein only the rinnegan can acutually allow capapble use of all elements which quote no single person can have as well yamato briefly said there is no shinobi he heard of that can use all five but your point on the other factors are pretty valid and I agree with. ButI allow people with a pretty valid kekkei genkai to use two elements at a early stage like kimmimaru's kekkei genkai is that of a physical one. By manipulating the calcium mass through the osteoclats and osteoblasts of his bones he can regrow and use them to his will a bloodline limit that uses it's physical properties. But since his clan focuses on mainly taijutsu and not ninjutsu we are not completly certain that he is not capable of mastering two.

basically people are capable or learning more then one element but this takes time for normal people without kekei genkai's to learn. But people with kekei genkai's have an advantage over this ability to learn more then one element at a younger age or at a earlier rank dont really know why but this has been proven over the course of the series. But just like everyone in the guild they can learn a third element when they reached a Kage level rank.

Cody places the card between his hands and focuses his chakra as the card becomes soggy and then splits in half in his hands.