If your pounding super fast double-kick chops on a single bass drum the sound can get all cluttered because of the hurricane of air inside the one bass drum.
With two, the sound tends to be clear in recording.
...and depending on who you ask, with double pedals there is always the lag on the slave pedal, metal heads with two kick drums have two single pedals which can be adjusted more easily and while each feel responsive.
If your for sound: Two Kick Drums
If your for tight spaces and logistical ease: One Kick Drum
But to tell you the truth, two kick drums don't take very much space on the stage (if the space is too small, why would you be there in the first place?), it is how you transport the kit that matters.