FYI, I contacted uAvionix support. They said that I can feed the SV-GPS-2020 serial data directly into the echoUAT via pin 6. The echo will send traffic and weather data to the EFIS from its pin 3. They did not recommend depending on the EFIS to communicate position data to the echo. As long as all the baud rates are set properly, this arrangement should work. The wiring diagrams in the echoUAT manual hint at this, but don't show it explicitly.