The multi-camera hardware sync feature of the Intel® RealSense™ D415 cameras is now available via the following firmware, v5.9.14.
This feature does not yet work on the Intel RealSense D435 camera. Support for the D435 is coming in the future but we do not have a timeline yet.
Note that this is development firmware so some features have not been fully validated by Intel. Do not use this FW in production builds.