Opening 2 parks at once is a heavy lift in terms of employee training, transportation infrastructure adapting to changes, and all sorts of other matters (i.e. making sure the technology all works, etc.); can it be done? Yes, but it still sounds like they'd start the building process for the water park and aim to open it the following year after the dry park.
I actually like this timeline:
2022: CityWalk 2.0 mostly opens, 2-3 hotels open
2023: dry park opens, 1 hotel opens
2024: water park opens, 1 hotel opens
Of course, VB will be getting slammed in 2022-2023 under this timeline, but I still think they'd prefer to stagger the openings in a way that Universal can train employees, add buses, etc. over a rolling 3 year span instead of mostly in a single year.