Musannaf Ibn Abi Shayba 26:337

Ibn ‘Ubadah narrated to us, from Muhammad ibn ‘Abd al-Rahman al-Gharbi, from Abu Ja’far, regarding the hermaphrodite (al-khuntha): He is to be inherited from based on his urination. If he urinates from both of them together, then [it is determined] by whichever of the two he precedes with.