Dual inputs are for those situations where you will use the R&L preamp outs and use the sub's internal x-over usually. In every case I can think of those are both summed to mono, since the sub is mono. You don't need to use both inputs, and on some subs, one or the other is the "preferred" one when you use a single input (check your manual). Some subs need a certain amount of input voltage (due either to input sensitivity of the sub or output voltage of the receiver) to remain on when something is playing that does not require much sub activity; in these cases using a Y can give you a little boost in voltage at the amp so it stays on (which can also similarly be done by increasing the sub's level in the receiver).
