watchOS 5.1 adds the ability to answer Group Facetime calls. The feature is audio only but you can switch to using video on your iPhone. Additionally, watchOS 5.1 introduces a new Color watchface that lets you select a solid color as the background.
tvOS 12.1 contains bug fixes and improvements but doesn't appear to include any new features.
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Developers can download tvOS 12.1 beta 4 and the watchOS 5.1 beta configuration profile from the link below.