Do you mean to feed both the left and right subwoofer inputs from the single subwoofer pre-out on a receiver? If so, that adapter will work but then you need two additional cables.
What you really want is the opposite of that adapter. You want what is commonly referred to as a 'Y-splitter'. It would have one female RCA plug on one end and two male RCA plugs on the other end. I couldn't find one on the monoprice site but lots of places will sell them, especially Radio Shack.
You can't double the power to speakers with extra wire - you turn up the volume on the receiver.
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