Hopefully someone can offer you the solution you're after (I'll be watching this closely!).
I would like to suggest you take a look at what you're trying to present and make sure it's the best way to express your data. I think that overlaying one on top of the other might be a little visually confusing for the viewer: it may not be readily apparent which bubbles are Q2 and which are Q3.
What if you plot the bubbles with Q3 values, and assign the color as the % difference Q2-->Q3?
The exact metrics can be listed in the Tooltip and/or Label.
Your user will probably be able to see which are the heaviest hitters, as well as which have changed the most.
Unfortunately, you'll lose any data you're currently associating with color (like [Category]) and I realize this may be a game-changer.
As another alternative: if you have the data in smaller segments than quarters (daily sales), you COULD use the pages functions to animate from one quarter to the next. I've never tried this with a bubble chart and I don't know what kind of effect you'd get, but it might be fairly smooth & fun animation that shows growth!