Have you actually installed and tried the Twinhan.
I have one (the Hybrid D+A) which doesn't need connecting to the sound card as it's done through the system.
However, the cable is required if I listen to the FM radio stations (there's a socket that accepts it at the rear of my SoundblasterLive! or I could use one of the Auxiliary equivalents.
But normally this cable is disconneted otherwise I would get two lots of sound on Freeview TV or radio...:-)