Dual svs SB12-nsd vs 1 PB12-nsd

My friend ask that same question to SVS and they told him that dual SB12's are louder until ~32hz within 3db@25hz and -5 or -6db@20hz than a single PB12. If this chart is correct then dual sb12's should be around 97/98db@20hz
http://www.data-bass.com/data?page=s...source=message

That is correct - a single SB12-NSD can hit ~92 dB @ 20 Hz @ 2M GP per CEA-2010 measurement protocol.

Dual SB12-NSD will have more mid/upper bass slam/impact than a single PB12-NSD.

A single PB12-NSD will extend deeper and will have more clean output at the very deepest frequencies (18-25 Hz).

Which option is best for you really depends on your particular application (room size, listening preferences, and overall playback level).

PMFJI. Why is this?

Something to do with having two subs or is it something specific about these subs?

Much of the lower frequency output on ported subs come from the ports resonating, not the drivers. Its pretty difficult for drivers to generate low frequencies by themselves, although some of the heaviest and largest drivers don't do so badly in low frequency output- but those are 18" drivers with enormous excursion. In higher bass frequencies, ported and sealed subs become more even in performance (all other things being equal), as the port contributes less output in that region.

Perceived bass slam is often heard in the upper bass frequencies of 40-80 Hz

In many cases, the output of a sub in these higher frequencies is usually limited by the amplifier watts. Having two SB12 with double the amp power will give you more headroom in those frequencies. The larger box of the PB12 is generally more efficient at all frequencies but generally speaking, ported designs only strongly beat sealed designs in output near their tuning frequencies, so even a single SB12 is not *too* far behind a single PB12 in the upper frequencies above say 50Hz