What did you think happened?
It was never some sort of death duel, people are forgetting this for some reason, or yall have some really linear ass idea of what a conflict is.
Shanks was trying to get to Marineford, Kaidou was trying to stop him.
Probably Shanks broke through some sort of blockade that Kaidou had set up, or merely shook his dudes off his tail.
All accounts so far make Kaidou seem like some sort of bad dude, so I really damn well don't see him and Shanks coming to some agreement.