Step 5: Final result
You can optionally insert a spout near the bottom of the first barrel. This barrel should be high enough off the ground the permit a bucket to be placed under the spout. If you also want a spout on the second one, the whole setup is going to have to be raised unacceptably high off the ground. With a previous rain barrel that we had, we found that the spout filled the bucket too slowly. That was true also if we connected a hose to it. With the current setup, my wife said that she would prefer that the barrels be kept lower off the ground and she simple takes off the lids and dips water out of them. This makes it a much faster operation.
Although I haven't done it yet, will somehow fixed some sort of netted material (such as the bag that onions are sold in) over the openings of each barrel. This is to prevent debris from going into the barrels and to prevent mosquitoes from breed there. There is no easy way to do this. I thought of using velcro to hold the material in place.
You will notice that there is no mechanism for letting the second barrel to overflow in a controlled fashion. Note that I have calculated that a tenth of an inch of rain will fill one barrel. If you get a half inch of rain, the second barrel will definitely be overflowing. For my arrangement, the water simply flows over the top and down the sides. Since our house is built on a slab, this is no problem for us. On the other hand, if you feel there is a need to lead the excess water away from the house, just drill another hole in the second barrel and use a similar setup to direct a longer PVC pipe out further into the yard.