Don't Expect Everything at the Same Time
You can expect Apple to release a watchOS 6 update at the same time as iOS 13, but we'll likely see a delay for Mac users.
Last year, Apple staggered the iOS/watchOS and macOS release dates. iOS 12 and watchOS 5 arrived on September 17th while macOS Mojave arrived a week later on September 24th. This is the same protocol the company used in 2018.
That said, don't expect Apple to stagger the iOS, macOS, and watchOS betas.
We fully expect Apple to launch pre-release versions of the new macOS and watchOS operating systems this June alongside the iOS 13 beta.