Rainbarrels connected from above.

These rainbarrels are connected at the top, by screwing 2" pipes into the  threaded 2" "bung" holes.  
The barrels fill in sequence--when one is full the rainwater overflows into the next.  
Each barrel drains independently through a hose connector at the bottom of the barrel.
Both barrels are strapped to the building for safety. The overflow (on right side)  sends excess water
back to the ground where it was going before rainbarrels were installed.
(Overflows should optimally be routed into mulch basins with plants whenever possible.)