Other forms: killing two birds with one stone; killed two birds with one stone; kills two birds with one stone
If you can combine returning your library books and meeting a friend for coffee in one trip, you've managed to kill two birds with one stone, which is pretty efficient.
Killing two birds with one stone sounds kind of violent; who'd want to kill birds anyway, unless they were your only food, or kept eating the seeds out of your field as soon as you planted them. That's where the expression comes from. But if you think of it as getting two things accomplished at once, that's something that will save you time and energy, and it's well worth your while. It can also be good strategy in a contest of any kind.