I'm trying to figure out how to easily do this with two barrels myself. I have 2 55 gallon drums, and I can make the overflow go into the 2nd one. But when I get a pump (I was using gravity last year) I can only pump out of one barrel at a time. I suppose I could just live with it, though I've been contemplating just having a connection at the bottom of both barrals to connect them together, has to be good and water tight though.
Just had a thought.... Run a hose up and over between the two barrels. Once they both fill up with water the hose will be full of water (might need some help the first time) and as you drain the second barrel it will create a siphon to bring water from the first. As long as you never drain the barrels completely and the hose goes near the bottom of both barrels it should always keep the level the same between barrels.