Found out about the Declates Crusade today.

"As a symbol of solidarity against what seemed to be an innumerable foe, Astartes from both Chapters "exchanged" iconography and often fought in mixed squads. Black Templars painted their left hands red, and Crimson Fists painted a Templar cross onto their right knee. This unusual gesture of unity helped establish a tight bond that steeled the resolve of the brave warriors involved."

Huh, didn't know that.