I'm an experienced S/W engineer, but new to Apple and wish to distribute my app for beta testing to remote testers with Windows PCs (no Mac's).
I know other beta systems (Crashlytics) don't support watchOS, so am asking the Q before delving in to App Center.
In principle, this could work, yes. However, Visual Studio App Center at this time does not yet support building and signing with multiple provisioning profiles so you could not use the cloud build feature to actually build your app.
At the same time, if you build your app locally or with some other CI solution, it's absolutely possible to upload and distribute an app that contains a Watch app through App Center. The watchOS app is simply bundled with the iOS app and installation will be handled on the phone.
To do that, you need to export your app, sign it with an development, ad-hoc, or enterprise certificate and provisioning profile and then upload it to App Center.