Just before the song One Of Us Simba's mane was blowing about in the wind but Kovu's mane did not why?.

I'm guessing it was because Simba was alot higher up then Kovu, so the winds were più higher up, so only affected Simba, o it was to make Simba look abit più dominereing and make him look like he's got the most authority.

maybe because of the altitude that simba was at and maybe to make the scene più dramatic

Because they were facing each other. Simba was facing in the direction where the wind blew against him, therefore making his mane blow. Kovu was facing the direction where the wind was blowing. Maybe it's the other way around.