The term "kill two niggas with one bullet" is nothing more than a spin off of "to kill two birds with one stone." Both of them simply mean that one is able to accomplish two tasks while only exerting the energy necassary to accomplish one. The term comes from the extreme violence that has plagued the gangsta rap community since the early 1990's. While people from the old days would kill two birds with one stone, the people of today kill two niggas with one bullet.