Two drivers in one enclosure are not the same as "two subs". It is one sub utilizing the combined output of two drivers and a single high powered amplifier. This isn't car audio where simply cramming a bunch of drivers into the rear firewall will solve all your bass problems. Bass is an audio signal just like the midrange and the treble. Details like transient response, THD, group delay, phase, driver/enclosure alignment, tuning, frequency response, and such affect the accuracy and realism of the bass reproduction the same as those things all affect the midrange and treble.
So, don't be fooled by the archaic and short-sighted quick fixes often employed to make the neighbood rumble when building a car stereo when looking into accuracy and realism in a home subwoofer.
The PB12-Plus/2 is a single subwoofer. It produces 6dB more output than the smaller PB12-Plus which uses a single woofer identical to the Plus/2. They offer nearly identical frequency response performance, but the THD is cut by as much as 6dB by using two woofers on the Plus/2. The maximum output in increased by 6dB SPL. And the transient response is improved by having more motor mass with two drivers.
Whereas, using two PB12-Plus subwoofers will only increase the maximum SPL by 3dB over a single unit. The THD will only be cut by 3dB. The transient response will be better, but not nearly as much better as with the PB12-Plus/2.
These are all defined by the laws of physics. Hence my advice to use one better sub rather than two lesser subs if you want the best performance.
Of course, there is financial, or performance, point where this advice is not the best. If you are getting into series stereo bass with two excellent subwoofers, you wouldn't want a single larger sub and sacrafice the stereo bass. There are always exceptions to most rules of thumb.
