Sayyiduna Zayd bin Thabit (may Allah be pleased with him) said: When two grandmothers come together, the sixth will be between them. When the one from the mother's side is nearer than the other, the sixth will be for the nearer one; and when the one from the father's side is nearer, the sixth will be between the two of them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الفرائض / 12363