I daresay Andy's right in modern, multi-tasking terms, but the original was surely, "You can't kill two birds with one stone."
The idea was that, once you'd thrown a stone at one bird and killed it, it was highly unlikely that the stone would go on travelling and kill another one in the same shot. In other words...concentrating on one thing at a time is the best way to get everything done properly.
For the record I agree with Quizmonster this is mainly to settle an arguement. I always thought saying oh I just killed 2 birds with one stone was a way of DEFYING the phrase. Can't makes more sense it's like you can't cut corners in life.