A number of people have dual subs for the extra umph.
The easiest thing to do is put the 2 subs right next to each other. This way you dont have complex peaks/valleys in your room from 2 separate sources.
If you put the subs in different locations, you now have to try and adjust the phase control to try and get the subs in sync at your listening position. But.. this does not promise the other seats in the room dont have problems.
Search the Speakers fourm for "dual subs" for some other discussion.