Sheikh Bandar b. Muhammad al-Rabâh
This type of situation occurs more often than people think. Natural disasters like floods and earthquakes are often the cause. Then there are house fires, automobile accidents, and other tragedies where people who are closely related to each other and have mutual inheritance rights often die together. This is why we find that the books of Islamic inheritance law often devote an entire chapter to the subject of multiple inheritors dying at the same time.
In some cases, it is possible to determine who died first. When this can be determined, the estate of the first person to die is distributed to all of his inheritors, including the co-deceased. This portion is added to the estate of the co-deceased. Then, the estate of the co-deceased is, in turn, distributed to his inheritors.
Scholars are in agreement about this procedure.
Scholars disagree about what to do in cases where it cannot be determined who was the first to die. However, the strongest view seems to that there is no inheritance between them. The distribution of their respective estates is calculated without factoring in the co-deceased as one of the inheritors. This view was adopted by Ibn Taymiyah and a number of other later scholars who reviewed the issue.
Therefore, if a man and wife die simultaneously in a car crash, like the case you ask about in your question, the man's property will be divided out among his inheritors without his wife being counted as one of them. Likewise, the woman's estate will be divided out among her inheritors without her husband being factored in as one of them.
And Allah knows best.
Source: Islam Today