On Wed, 31 Mar 2010 18:24:49 +0100, Ed wrote:

1. Connect from top of barrel A to top of barrel B
2. Connect from top of barrel A to bottom of barrel B

A will fill to the take off point for B. B will fill to it's overflow
level. A will fill first then B. When B is empty you'll have to go to
A for water.

3. Connect from bottom of barrel A to top of barrel B
4. Connect from bottom of barrel A to bottom of barrel B

A will only ever fill to the overflow level of B. If B is entirely
below A, A will never fill. You can take water from B all the time
but may have less water to start with.

I'd connect top to top as the flow into the bottom may disturb the
sediment that will accumulate or draw it into the pipe. You can also
clear the connection pipe without having to empty either barrel.

--
Cheers
Dave.