PX4 Guide for RTK

PX4 FirmwareGPS Heading with Dual u-blox F9P is supported in the both DroneCAN and UART version. The DroneCAN version requires a different firmware and can be found in the guide below.
RTK GPS Heading with Dual u-blox F9P | PX4 User Guide
RTK GPS | PX4 User Guide
Holybro H-RTK ZED-F9P RTK Rover (DroneCAN variant) | PX4 Guide (main)

Ardupilot Wiki for RTK

Ardupilot GPS Heading with Dual u-blox F9P is supported in both the UART and DroneCAN version.
GPS for Yaw (aka Moving Baseline) Copter documentation
GPS/Compass (landing page) Copter documentation