Halakhah for Sanhedrin 90:2
ואם לאו עד השני נוטל את האבן: נוטל והתניא רבי שמעון בן אלעזר אומר אבן היתה שם משוי שני בני אדם נוטלה ונותנה על לבו אם מת בה יצא וליטעמיך תיקשי לך היא גופא משוי שני בני אדם נוטלה ונותנה על לבו
But has it not been taught: R. Simeon b. Eleazar says: 'A stone was there which it took two men to lift, — he lifted that and dropped it on his [the victim's] chest; if it killed him, his duty was fulfilled'?<span class="x" onmousemove="('comment',' Obviously two people were required to handle it. ');"><sup>3</sup></span> But on your reasoning, that itself is inconsistent! That 'which it took two men to lift' — 'he lifted that and dropped it on his chest!' But it must mean that he lifts it up together with his fellow witness, but drops it [down] by himself in order that it may come down with force.<span class="x" onmousemove="('comment',' Because if two threw it they might not both follow exactly the same direction with a consequent loss of force. ');"><sup>4</sup></span>