If Kopa was in the TLK || then what would happen to Kovu since two male lions can't be in the same pride?

king kovu should change the rules and it should say there can be as many male lions who wish to be in the pride

I couldn't answer that, because he never existed in the movie universe anyways. so I do not know. The movie could never change different, with a character from the books that were never created by Disney It's been 14 years since TLK 2 was released, anything like that can't change. I actually never heard of Kopa since last year and he only exists in the books, he was never Fluffy nether. Kopa would never be in any films when he's not an official character.TLK films are good the way they are.

If Kopa - or any son of Simba and Nala - was in the story, I don't think they'd written Kovu in it at all. After all the most basic point of Kovu is that he falls in love with SImba's daughter after he joins SImba's pride, so that he could become who he can be instead of who his mother was trying to make him.

So what would happen to Kovu if Simba has a son in the sequel?
My answer simply put: Kovu wouldn't exsist in the first place and the story would be perhaps entirely different from what it is now.

But that's only for the story matters.
Because lion prides have one or two LEADER males, but it doesn't mean there wouldn't be more males in general. A lion pride may have even 6 males. So in that sense Kovu wouldn't necessarely be a problem. But just for the matter of the story Disney wanted to tell.