You are assuming it is the same dev team working on both of those .
You do know they have multiple teams working on different aspects of an expansion , in fact they also have multiple teams working on multiple expansions .
https://gamerant.com/world-of-warcraft-the-war-within-midnight-simultaneous-development/
World of Warcraft recently revealed it is doing work on both The War Within and Midnight concurrently. According to general manager John Hight and executive producer Holly Longdale, there are three teams working on [World of Warcraft] simultaneously: one on each expansion, and another on the current updates to the live service game.