Doing some going back through old postings and ran across this one. We too added a couple of rainbarrels this year, after all, we might as well take advantage of the 'Wet Coast' moniker and we ended up using a locally manufactured diverter in conjunction with 2 barrels. We were fortunate in that the city actually offers the needed bits and pieces at a subsidized price. You can see the handywork
towards the bottom of the linked page to get a better idea of how it works.
Since that picture was taken I have done some slight re-arranging of the kit-n-kaboodle. It turned out that with the staggered barrels I had an issue with overrun. The higher barrel would always want to drain into the lowest point (duh) and of course that was a bit of an issue. What is supposed to happen is that once the barrel reaches a certain level the water would remain standing in the diverter tube and the water would run back into the downspout again. That never happened, so periodically I would have to put a plug into the diverter because I would have 2 full barrels already. Now the 2 barrels are at the same level, it's been raining of and on over the past week and I have not seen any overrun as yet, but then again, I haven't looked very closely either.
It is amazing by the way how quickly these barrels fill up and how quickly you can go through 100 gallons of water even when you handwater only the pots and baskets. There were many days over the hotter parts of the summer where we ended up having to resort to city water again because both barrels were empty. I think we'll find a way to add at least another 2 for next year, the water will not go to waste.