Maymun bin Mihran (may Allah have mercy on him) said that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: When a man has two slave-women who are sisters and he has intercourse with one of them, he must not go near the other until he removes the one with whom he had intercourse from his ownership.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13940