They’ll both lose their Road Poneglyphs, I think everyone knows that.
I suspect that at least one of them won’t completely lose, and something will have interrupted the fight.
If I was to guess, Shanks outright beats Kid, Blackbeard is beating Law but the Marines or something interfere and the fight isn’t concluded.
The way the fights will probably go, we’ll get a start to Kid vs Shanks then we’ll not see any further. Think it’ll be the end of Egghead, we’ll get the usual paper exposition. “Oh wow, says Blackbeard and Law fought on Winner Island, looks like the Marines interrupted it.” Next time we see Law, he says he lost his Road Poneglyphs.
Then we’ll get to Elbaf with the Strawhats and the reveal will be Shanks beat Kid.
Mostly because I think Oda will find it a bit boring to have both beaten outright. He also needs to make a reason for Blackbeard not to just kill Law. Easier to interrupt the Blackbeard fight- Garp’s already off to Fullalead. Blackbeard is still getting stronger, his bounty shows we aren’t at peak Blackbeard yet. And we’ve seen a good bit of him. Like, it’s not surprising that Blackbeard got injured by Law, he always gets injured.
For Shanks though, this is his first proper fight. Can’t get dehyped there. He’s a nicer person than Blackbeard, so isn’t expected to kill Kid. Because it’s on Elbaf, home of pride and haters of Big Mom, it’s an easy plot to say they were impressed by Kid’s spirit and him beating Linlin so let him stay there to recover. Unlike Blackbeard, Shanks probably isn‘t getting any stronger so needs to look good in his prime. It’s just more expected by me he comes out looking best.
And of course, Blackbeard‘s main hype moment in the future will more than likely be beating Shanks anyway. So for that to work most effectively, Shanks has to look incredible until that moment.
What actually comes of Kid/Law after their fights, tbh, don’t care, they simply aren’t that interesting. Seen enough of them both.