Water Butt Link Kit
From: Watering and Irrigation : Waterbutts & Accessories
Efficiently link water butts or other containers together to increase water storage capacity using this easy to fit Water Butt Link Kit.
Two internal nuts, sealing washers and connectors to securely fix the 10¼" (26cm) linking pipe supplied between two water butts.
To fix the link kit in place you will need to drill a 25mm hole in each of your water butts.