When driving a set of doubles, you need to close which shut-off valve in the last trailer:

Shut-off valves are used in the service and supply air lines at the back of trailers used to tow other trailers. These valves permit closing the air lines off when another trailer is not being towed. You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed. [California Commercial Driver Handbook]