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.

Surround content mix:

50% Center channel
40% L&R channel
10% Surround channels

The center channel is the most important speaker. Typically it is smaller than the L&R. A sub would help give you full range response for center channel information. In 5.1 processing each channel can receive info down to 20hz. The sub .1 channel gets special effects info that is not the same as what goes to the other 5/6/7 channels. The sales guy is giving you good advice if you have a smaller center speaker.

However, the system is now more complex. Set-up will require the sub is time aligned with your center speaker. You need another cable possibly a crossover, another amp, power cord and a place to put it. This is basically a sub/sat system for the center channel. Weigh this against buying a full range center channel that matches your L&R in the first place.

If you really want the best sound you should have identical FULL range speakers for all 5/6/7 channels. This is usually difficult due to speaker placement constraints for center and surrounds. Full range surrounds can be hard to justify as you are spending a lot of cash for that 10% of sound.

hope this helps,

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.