If the sub doesn't turn off when you are watching movies or listening to music, there's no need to use both inputs because some subs need a certain level of voltage to remain on. If it bugs you, use a Y female to 2 male splitter, but the end result is basically the same because the signal is recombined inside the sub and both inputs are receiving the same mono signal already. There are two inputs because the sub can be used with a full range stereo.
