if you're not driving your system at or close to maximum volume, you won't need the second B1
That quote from iibw is really what its all about. Very well put.
On the spectral balance: By spectral balance we mean the correct proportions of lows, mids and highs. This is what we try to maintain.
Let's look at an example. For the sake of argument lets assume we want equal amount of level at 100Hz and 1 kHz. Say its 90 dB in the room. If you add a second B1 (without compensating) your acoustical level at 100 Hz would go up to 96 dB (because you double the acoustic volume for 3 dB and you double the total electrical power for another 3 dB). That would sound too "bassy". To compensate the PS1 drops the electrical output by 6dB so that the acoustic output goes back to 90 dB where it belongs in the first place.
The only difference between one and two B1s is in the maximum acoustic level you can get at 100 Hz. Lets say you crank the systems playing bass guitar. At a certain volume level (say 105 dB) the B1 is out of steam. The limiter kicks in and would prevent it from getting damaged or sounding bad. But the L1s still can go louder (since there isn't much high frequency energy in the bass guitar signal). Now if you add the second B1, you can still keep increasing the low frequencies to about 111 dB. Up to 105 dB output the 1 and 2 B1 configurations would sound the same. Only at high levels there will be a significant difference.
It not only depends on how loud you want to play but also on your input signal. If you are a singer, you really don't need a second B1, because the bulk of the energy is in the mid and high frequencies and the L1 will reach its way before a single B1 is maxed out.
I hope that all makes sense to you. If not, please post back and I'll try to explain it differently.
Nothing in the world is perfect, so 1 B1 and 2 B1s will never sound "exactly" the same although it should be pretty close. There will always be some minor differences depending on the room and where you are standing. Sometimes is just that the second B1 (stacked on the first) is a little closer to your ears and feels louder at the playing position. There is also more headroom, i.e. at any given output level 2 B1s don't have to work as hard as a single B1.
Hope that helps