The Gaze SDK can connect to multiple trackers, but the EyeX Controller and Steelseries Sentry device drivers only support one of these eye tracker to be connected to the system at a time.
About the relationship between the Gaze SDK and the EyeX Engine, it is true what Edward wrote that the Gaze SDK is completely independent of the EyeX Engine. But note that since we are bundling the EyeX Controller drivers with the EyeX Engine software, it is very likely that the engine will be running on your end users machines and that the users will have completed screen setup and calibration procedures using the engine software before running your application. The EyeX Engine will automatically connect to the first eye tracker found on the system. Which eye tracker it will connect to probably depends on the order the USB ports are “scanned” (I don’t really know the proper term for this 🙂 ) for connected devices by the system.
The EyeX Engine is actually built on top of the Gaze SDK, and uses the Gaze SDK to connect to the eye tracker.