|Neither of them would work because they would be competing with each other to synchronise with the DSLAM at the exchange. As soon a you plugged the second on in it would disrupt the synchronisation of the first and neither of them would synchronise.|
The Modems have to synchronised with the DSLAM before any communication with the ISP can take place. That is what the orange flashing light on the Modem means; synchronisation is tacking place. Once synchronisation is complete the orange light becomes steady. Once communication is established with the ISP the light will change to another colour or another light indicating an Internet connection is available depending on the Modem.