Is one period of Istibra' sufficient if a female slave was jointly owned and subsequently sold?

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If a female slave (amah) belongs to two masters who both have intercourse with her, and subsequently they sell her to a third man, a single period of Istibra' (waiting period to ascertain purity) suffices for the buyer. This is because the purpose, which is establishing freedom from pregnancy (bara'ah), is achieved thereby. The necessity of Istibra' for a manumitted female slave is predicated upon the act of sexual intercourse itself. If she is manumitted while she is someone whom the master does not have intercourse with, no Istibra' is required. Since intercourse occurred from two parties, the consequence of their intercourse is binding. In the case of a sale, the obligation for Istibra' is predicated solely upon the renewal of ownership, and nothing else.