That's different from a hotzone - "hotzones" are what the SIMILE Timeline folks call parts of the timeline that have a different time scale than the rest of the timeline. You can see an example here:
http://simile-widgets.org/timeline/examples/jfk/jfk.html - the main timeline has a month scale, but in the middle it changes to a day scale and then an hour-by-hour scale.
Is the issue you're describing that the white band of visibility on the lower band gets smaller as you increase the intervalPixels on the top band? This is, as they say, a feature - the lighter band on the bottom shows where you are in the larger timeline, so if you increase the size of the top timeline, thus narrowing the visible range, this area on the bottom will also grow narrower. If you don't like it, you could fix it by increasing the intervalPixels of the bottom band as well.
I hope that's helpful - let me know if I misunderstood your question.