Foster, OK then my answer is "not necessarily". Which way is actually "best" I think depends on the sub's selected low frequency cut-off point.
For filling in low frequencies up to 50Hz, one sub is better. Since the brain can't localize a source below about 80Hz, placement isn't that critical as far as imaging is concerned.
Above 100Hz, two are definitely better than one. Between 50 - 100 Hz it's a tough call, but I'd still start with one. Then, if there's insufficient low-frequency imaging (meaning the bass doesn't seem to come from the mains) then two subs placed somewhere near each main will provide more realism -- but then room acoustics will require more adjustment.
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