Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second OS X 10.11.2 El Capitan Beta to Developers
The new beta, build 15C31f, can be downloaded through the Apple Developer Center and through the Software Update mechanism in the Mac App Store.
It is not known what improvements the second update to OS X El Capitan will bring, but it’s likely to focus on bug fixes, security enhancements, and performance improvements to address issues that have been discovered since the release of OS X 10.11.1. Apple's beta 1 release notes asked developers to focus on Graphics, Mail, Wi-Fi, Calendar, USB, Notes, Photos, and Spotlight.