I am a huge re-cycler and try to find new ways to re-use just about everything. Today I am posting a picture of my rain barrels. One was store bought and the other was made from items we had on hand. Between the two we can gather 110 gallons of rain water to use on our flowers and garden. I am a firm believer that God's rain is so much better for our plants.
These work perfectly. #1 has a valve for releasing water and on #2, I simply dip in watering cans and fill.
We bought it at a place called Greenway Supply here in town. They supply all sorts of things for people wanting to live more naturally. I am sure most major cities have a similar store.
You could use this same system with 2 homemade barrels. You would just need to place a section of hose through the side of the top area of one (I would silicone around that), then have the over flow go into the other one.
Hope this helps you decide to start saving water. It seems so much better for the plants, and best of all the water is FREE!!!!