Musannaf Ibn Abi Shayba 18:1243

Abu Bakr narrated to us, he said: Mu'adh ibn Mu'adh narrated to us, from Ash'ath, from al-Hasan, regarding two men who wounded a man: One of them wounded him with an *amma* (a head wound reaching the membrane covering the brain) and the other wounded him with a *mudihah* (a head wound exposing the bone), and it is not known or it is not understood which of them inflicted the *mudihah* and which of them inflicted the *amma*. He said: Upon each one of them is half of the *amma* [compensation] and half of the *mudihah* [compensation].