These two will be part of my Naruto AU but I plan on giving them quite a different role than Otsutsuki have been given during the series (as I seriously fucking hate Boruto as well LOL).

In short, Hiyaku (the younger one) is obsessed with life on Earth and escapes to the planet he's obsessed with, Yuuto tried to catch him and chased after him but is unsuscessful, so he ends up on Earth as well with Hiyaku.
Once on Earth Hiyaku decides to sate his curiosity about everything (and everyone) and Yuuto, less curious but still good natured, protects him and tries to get him to not to fuck up 24/7.

Yuuto is very well acquainted with most of the Earth's customs, such as words and terms, but Hiyaku isn't.

They're both good natured "people" who mean no harm and will be used to enrich character interactions, so nobody needs to worry about them utterly destroying the plot lol.