15He Can Use Two Nature Elements Simultaneously
Yes, Naruto has used two nature elements simultaneously before. However, he relied on chakra from the tailed beasts to accomplish those techniques. Also, let’s not forget that he couldn’t do that until he acquired the Six Paths Senjutsu.
Despite his age and the lack of chakra from a tailed beast, Boruto has already
learned how to combine two nature transformations together and use them simultaneously while attacking. His attacks on Naruto were so strong during training matches that Naruto was forced to put up a protective wall to handle the assault.
By combining these transformations together at the same time, a completely new elemental nature is created with its own unique properties. A simple example of how this was used in the past would be Hashirama Senju’s Wood Release, combining Earth and Water natures.