To use only one action to complete two tasks.
The only true way to kill two birds with one stone literally is to have the following involved:
1)"Kill" two flightless birds with one very big stone. 2) Use one stone but recover it after each shot until two birds are "killed". 3)Wait until two birds are in close proximity and fire the stone from a bazooka. 4)Meteorite collision with Earth.
We can kill two birds with one stone if we go to walmart and get what you need as i get what i need.