The new macOS High Sierra 10.14.2 beta can be downloaded through the Software Update mechanism in System Preferences after the proper profile has been installed from Apple's Developer Center.
We don't yet know what improvements the second update to macOS Mojave will bring, but it is likely to include bug fixes and performance improvements for issues that weren't able to be addressed in the macOS 10.14.1 update.
No new features were discovered in the first two 10.14.2 betas, but we'll update this post if anything new is found in the third beta.
macOS Mojave 10.14.2 comes shortly after the release of macOS 10.14.1, an update that brought support for Group FaceTime on the Mac and introduced new emoji characters.
Update: Apple has made a new public beta of macOS 10.14.2 available to its public beta testers.