i think you are right about the dual breaker connection, im only speculating cause i have no idea how your space is wired up but see this picture:
so both sides of the dual breaker are 'one half' of the center tapped side of the xfmr. On the other side of that xfmr, it is one source. SO maybe, if you consider the picture in my last post, losses in the wiring BEFORE the xfmr would indeed reduce the voltage of BOTH halves of the center tapped side, even if only ONE half of the center tapped side was responsible for pulling the current.
this may not be what's happening. its just a possible explanation.
as for why the lights dont dim when everyone is playing... id geuss that the connection from hydro kebek to your building is on a pretty fat wire. hoewever once its in your building, it may be distrubuted on smaller conductors that are more lossy, but sort of in a "star" configuration to smaller panels. So you pulling the voltage down half a volt or so in your space may not be seen on teh guy next door since that drop is happening over the wiring of your arm of the star, not over the link from hydro to your building. IE at the main entry point the voltage is not reduced. Again im just guessing, im not an electrician, and have little experience in that area.
In all likelyhood there is a main panel; that redistrubutes to many smaller xmfs in your building. at my workspace we have an xfmr half the size of a fridge right in the atelier for example.
EDIT: re why teh lights dont go out everytime slayer plays next door: it may also be as the pitcure shows, the input voltage to these xfmrs is way more than 120 or 240. Lets just say its 10 times mains, then 1 amp pulled on the secondary (mains side), is only .1 amps on the primary (high/distribution side). So even if the wiring guage was the same on the hydro kebek side, the losses per meter are 10 times less since the same power is available at 10 times less the current.. food for thought.