In the Quran, it says that one man is enough to be a witness. However, if the witness is a woman, there needs to be two women for the witnessing to be valid. Why is this so?
Witness is usually taken when there is a dispute between parties and conflict of interests. Witness is a heavy burden which can be difficult on one woman alone. Another woman supporting the first one will make the witness fully accepted.
In fact, it is out of respect for the woman not to leave her face the challenge of the witness alone.
Wassalam.