I will give this a "maybe" in response. Here is why one might be better than 2 (since the very valid reasons 2 might be better than 1 have been laid out above, in particular the emergence of stereo effects at 80Hz or so, and the potential of 2 subs to minimize room nodes):
1) The quality of the single sub is much better. I moved from dual NHT subs to a single REL Storm III, and this was definintely an improvement in my room. Related to the quality of the sub is the quality of the intergration a sub can achieve with mains. For the REL that came through their speaker level connections, but I could also imagine that single subs with DSP (digital signal processing) options could also work better than dual subs without DSP or with lower quality DSP.
2) You don't need a high crossover point. My NHT pair was originally purchsed when I needed an 80 Hz crossover point. My single REL was crossed over at 35 Hz.
1) The quality of the single sub is much better. I moved from dual NHT subs to a single REL Storm III, and this was definintely an improvement in my room. Related to the quality of the sub is the quality of the intergration a sub can achieve with mains. For the REL that came through their speaker level connections, but I could also imagine that single subs with DSP (digital signal processing) options could also work better than dual subs without DSP or with lower quality DSP.
2) You don't need a high crossover point. My NHT pair was originally purchsed when I needed an 80 Hz crossover point. My single REL was crossed over at 35 Hz.