When using the default method of discovery (mDNS), they systems will find each other using all network paths, which can result in Ethernet and WiFi being used if both systems can be found.

The best away around this is to run Discovery Server on one of the systems and configure NDI Access Manager with the setting. Makes sure the IP address you enter into Access Manager is the Ethernet IP address (not the WiFi one). Then NDI application will no longer use mDNS for discovery and they should stick to the Ethernet network for communication.