Hazmat placarding for 2 attached trailers

First off do not confuse placards with labels. The requirement for labels are those that go onto the package itself. Now the requirements for placarding are listed in the CFR Transportation 49 parts 100 to 177. Specifically 172.500 Subpart F-Placarding. Under 172.504 General placarding requirements through 172.523 you'll find what you need.

 

Here is a video of a Houthi (Yemen) Ammo Dump that was underground and hit by F-15 Fighters in a bomber role with munitions that reached down and turned the place into a rain of ammo and fire. You might want to calm the sound down, it's loud.