Because that "clash after" was never displayed. First panel was King already having a grip on Zoro's swords, next panel is them remaining in the exact same position without swinging their blades again, just with distance. With Zoro saying it's all about not letting his sword being taken away. Zoro's eyes in those panels were a depiction of his reaction (widely open and exclamationmarks !!), while King's just looked determined.
The way I see it. It was initially an attack by King i.e., King tried to hit Zoro when he was vulnerable but he defended, and then it became a struggle, a struggle b/w Zoro trying to save his swords and King equally trying to take them away. As a result, they both used their wills to intimidate each other during that struggle, and hence the equal emphasis on their eyes. There was a similar emphasis on Kuri and Luffy before their COC clash:
As I said earlier, you can use COC to push things away but King didn't budge an inch. You can't apply pressure on yourself with your own COC—COC never worked that way. There was the focus on their eyes, which is something Oda usually does to indicate COC usage. If Zoro was the only one using COC, the black lightning wouldn't appear exactly in the middle. The waves either spread towards Zoro or in the direction of King(if it's pushing him away).
This is how COC waves appear when only one of the two people involved uses COC:
COC waves mostly only appear in the middle if there is a clash. We also can't ignore how identical the depiction was to WB and Roger's clash. This chapter is packed with panels mirroring past events so I don't think it was a mere coincidence.
