Mostly the above said is not really a good assesment of the Pro's and Con's.
Adding more subs(in this case two) can increase the obvious dyanmic range of a system, by adding more bass to the room potentially. At the very least, you can run two subs at less output than one single sub, for more dynamic potential and ease/effortlessness. That said, you can also run stereo bass(which I'm not sure your pre offers). This allows for each sub to only have to deal with Bass from the left or right channels, mixed with the LFE or .1 channel, rather than handling all the bass symultaneously.
Yes, you can smooth out potential bass mode problems by placing one sub so that it is placed to counteract bass peaks in a room, thus smoothing response overall, by not coupling well at certain frequencies, if the other sub is placed where it does excite certain frequencies. (i.e, one sub placed in a peak at say 40hz, and the other placed in the null at 40hz). When the two subs are balanced well, it evens out frequencies avaraged between the two better, rather if placed in different strategically calculated locations. Also, with STEREO BASS, you can get better potential bass separation, detail, and imaging, less distortion potentially, just like stereo woofers in a full range speaker...all to varying degrees depending on execusion, acoustics,effectiveness of set up, calibration, etc.
Basically, two subwoofers can offer a lot of advantages IF SET UP CORRECTLY! The problem here is that most would probably cause more damage than good from lack of knowledge of what they're doign if they don't implement things properly.
For instance, it's very easy to get phase cancelation from comb filtering. at certain frequencies(likely one or two different freq's), especially in "mono" dual sub configs. You can also put two subs in tandem "Peaks" at certain bass frequqencies, compounding bass modes. You can put subs at distances from each other and the wall boundaries that causes more bumps and valleys in the response as well, without getting into too many details there.(See "Haus effect/Shreoder effect", others)
Basically, I think people can't set up one sub well enough mostly, let alone two or more! But, in the right hands, dual or multiple subs can be very effective if done correctly. INfact you can do much better in the hands of a skilled person who's putting the system together. Two subs can help maximize out put, smooth response, increase range, separation, imaging, etc.
The key here is you gotta know what you're doing...otherwise, you're just adding more speakers without knowing what's happening