2nd sub for center channel ?

No need for a center sub. Set the center to "small" in your menu, and the sub will take care of the frequencies below the cross-over point.
I have a reasonably large center (Martin Logan Cinema), and still set it to small as Martin Logan suggests, because there is enough low frequency information that it makes a good difference.
Go from small to large from your preamp, and you'll hear the difference quickly.
There is more than dialogue from the center (movies), and if you want to prove that to yourself, just turn off the fronts and see.

Let me clarify my post above -No need for a DEDICATED center sub - by using the technique I mentioned above, the info. which is sub level for the center channel will be incorporated when it's in the small setting.
As most TV's (and rooms) can't support a full range center as Steuspeed correctly suggests, setting the center to small will allow full range response by incorporating the sub.
While the .1 function of the sub does do the special effects function, incorporating the center channel's additional requirement is not substantial, and won't affect the subs performance.
Go back and forth betwen small and large on your processor for your center if you want proof.