I know this thread is old but I came across it whilst looking for something else...
It doesn't matter if you are using 1 4x12 or 2 of them the volume always stays the same; of course depending on the amps you are using (taking into account stereo rack amps).
However, when using 1 4x12 all the power is sent to that cab, when using 2 cabs the power is split between them. Again, this only works if you're using a MONO amplifier to power them.
I found something that should give everyone a laugh... I found on another thread the reason for having a bottom cab is: "The bass comes out the bottom cab and the treble out of the top"... unless they're using a crossover between the preamp & power amps it's not the case. Just sit in front of the bottom one and get blasted to hell the same as you would the top cab. The only difference being more bass if there is more wood in the cab (eg. Marshall 1960B) or different speakers/EQ applied.
I wish I knew where I found that thread. It was funny.
Anyway, when I was playing live with band (haven't played live in a bout 3 months!

) no matter how much I turned up my 50watt stack I was never much louder than the drummer! So the full stack always got mic'd up... we would mic up everything including the drums & DI the bass from his 350w Amp... being a Sound Engineer I was always obsessed with getting a decent balance. Most of the time the bass & drums were very low in the FOH mix because they were bloody loud on stage.
Why do I use a full stack? 1) I always dreamed of having one so I bought one (over many years)... 2) It looks cool! It's the ultimate icon of any form of Rock.
Did I always plug in both cabs? Yes. Why? because I couldn't hear myself at the best of times with just the 1/2!