What are the rulings regarding separation from a wife with whom marriage took place during her 'iddah period from a previous husband, and how is the 'iddah period calculated?

The waiting period (iddah) is an impediment to marriage. It is impermissible to marry a woman who is in her waiting period to anyone other than her husband. Whoever marries a woman during her iddah, whether knowingly or unknowingly, must separate from her immediately with the intention and act of separation. There is no need for a judge's ruling.

If he has consummated the marriage with her, scholars differ regarding her waiting period: The Shafi'is and Hanbalis: She observes two waiting periods (an iddah from the first husband, then an iddah for the second), because the iddah is an act of worship and multiplies with the multiplicity of intercourse. She completes what remains of the first iddah, then commences the iddah for the second. The Hanafis and Malikis: The two waiting periods overlap, and she observes only one iddah (considering the clearance of the womb).

If a pregnancy results from the second (marriage), she observes her iddah by delivering the baby first, then observes an iddah for the second (marriage). The woman is considered a stranger to the second husband, and nothing is permissible for him from her except what is permissible for a stranger from a stranger, because his contract is invalid.