Dual subwoofers: the concept may seem like something only tweaky audiophiles would do, but in reality, it may be just what your home theater needs to get the smoothest subsonic sound on the block.
It's About the Increasing The Output: New home design in the last couple of decades has produced a lot of rooms that have soaring high ceilings, and these rooms have huge air mass. Here's the part where we get a little technical: it requires significant sound pressure levels (SPL) to fill a room this big, and a single subwoofer may not be enough to do so, depending on the size of your room.
Adding a second one to give you dual home theater subwoofers gives you a way of increasing the overall SPL and giving you deeper, fuller sound. If you favor thrillers or blockbuster action movies, the difference will be incredibly satisfying. Ditto if your musical taste runs to full orchestras: properly set up, dual subwoofers will give you the sense of being front-and-center at Carnegie Hall.
And It's About Increasing The 'Sweet Spots': The other reason to add a second source of deep bass is to "tame those nodes!" as Alan Lofft would say. Nodes occur at 1/4 the length of any frequency from the distance to the ceiling, walls, flo --- hey wait a minute - I said this wasn't going to be a tweaky audiophile thing!
So let's just say that all rooms have peaks and nulls, and in some rooms, those occur at undesirable locations, like your wife's favorite chair. As a result, she may not enjoy a movie or music as much as you do. So in order to even out those nodes and create the same great experience you're getting, adding a second sub to offset a node might be just the ticket.
Placing both of them properly requires a little experimentation. Watch Ian Colquhoun's video on subwoofer set up here:
For more set up help, read Alan Lofft's Dual Subwoofers Tips here.