If I remember correctly, there is an option for a satellite connection, which may explain the "Dual connector" . I did not have the option. I removed the plate behind the tv in the living room and the stereo.
The dish guy disconnected all coax from the plate in the living room and connected the main input coax "Cable" to his sat coax, He connect his second tv output to the white (antenna coax) and the "Set-2 coax with a splitter. No coax are connected to the living room plate. basically he did the same thing your guy did.
Here what I did to get the tv/dish and my stereo and antenna to work with the dish connected to the outside cable connector.
Hopefully it is the same as yours.
Th 3 wires coming from the wall behind the living room plate is as follows:
The main coax (black)goes to the "cable" the white coax go to "Set-2" and the 3rd (black) coax go to " antenna".
So in order to get my dish to work and my stereo to have an outside antenna, I will need to:
Disconnect my stereo antenna from the splitter behind the stereo.
Run a new coax from my stereo antenna to the "set-2" port on the plate.