Basically, hooking up both inputs, doubles the gain by 3dB.
I use this to increase the call voltage on an auto-on feature. If manually turning a subwoofer on (old school), then you're good with just the one connection. I have three subwoofers hooked up and they're all hooked up via a single RCA connection..
Yes, it's a foreign concept to leave a plug unplugged but a subwoofer is a mono signal. And just like your light bulb only needs two wires, the subwoofer only needs two wires (one RCA plug) worth of input.
I have five "Y" connectors in the spare cable box and a boatload of appropriate, 2m subwoofer interconnect cables. Today, I'll receiver another "Y" connector (needed for proper gender connection) and another subwoofer cable so I'm using same length subwoofer connection cables.
Below are links to items that are in the mail and will be here when FedEx makes today's delivery."Y"Subwoofer cable
I find the above cables to be the "BEST" cable design: price/performance. You can get cheaper, you can get more expensive. I find the look, feel, build quality and if one has Prime membership, two day shipping is included, to be the best. And considering the price of gasoline, if one is a budget minded shopper, two gallons of gas adds to the cost of everything, real fast.
As to your question regarding parallel cables, in my case, I solved that problem by separating a single, side-by-side stereo cable into two separate mono cables and now I have two 8' RCA-RCA, male connector tipped cables.