Ibn Jurayj (may Allah have mercy on him) says: I said to Ata' (may Allah have mercy on him): I made a contract of manumission (mukatabah) with two slaves of mine and wrote that (each is liable for the other) upon them. He said: It is not permissible with your two slaves. Sulayman bin Musa also said the same. Ibn Jurayj says: I said to Ata': Why is it not permissible? He said: Because if one of them becomes bankrupt, he returns to being a slave who owns nothing of yours.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21634