Before supplying air to the air tanks of a second trailer, what do you need to do first?

A Open the shut-off valves at the rear of the first trailer and close the shut-off valves at the rear of the second trailer.

explanation

Open shut-off valves at rear of first trailer (and on dolly, if so equipped). Close converter dolly air tank petcock and shut-off valves at rear of second trailer. [Commercial Driver Handbook]