I think, that two subs are inherently better, than a single one, for many reasons:
Bass is directional, despite a popular opinion, than it isn't
Two subs will give you smoother in-room frequency response
Two are easier to position, to alleviate room- related modal issues.
When I was about to add subs to my own system, I have done some research and spoke with quite a few experts, and that's what everyone of them said.