Dual Subs

Have been using Tyler D3M's for several months now. Excellent monitors with my Plinius electronics. Have been using them with a DS12 sub.

I'll be educating myself on this over the next while, but thought to throw up a general query. What are the benefits of running two subs? Is there something that two subs deliver that one could never technically produce?

Reason I'm asking is that I am very satisfied with the effect of this sub in my room (room is 35 x 40 x 10 ft high). But I've always wondered what may be the benefit of adding the second sub. I've had one local shop tell me that if indeed you have hit the 'magic' spot, and you are delighted with the sound, the affect of the second sub may be marginal at best. Not knowing if true or not, now considering that route.

thanks for any and all opinions on this