KKH played 51 games last year, Milan played 48. They don’t need to be rotated every game they are both worth of being a comfortable first choice right back at this level, or higher.
And they could both play that many games this season.
Let's say we play 50 games next year. That's 4.500 minutes so 2.250 each, which is still a lot, especially if you want them to play at 100% all the time in such a demanding position. And there would likely be times where they play together, either one in front of the other or maybe one needing to fill in at LB.
I don't think you realise how fortunate we've been that MvE hasn't been injured much given the workload he's had over the past two season. It's how we wrecked Dabo.
I'll say again, it's not about one being first choice and one back up. Both would be a first choice with their workload managed so they give everything the entire time they're on the pitch.