Let’s assume your spouse is younger than you and you each own a small business. You want to provide for one another on death and so you leave your assets (including your businesses) to each other.
You have no children and your will states that if your partner is not alive your assets will pass to your parents. Your spouse’s will also states if you are not alive his/her assets will pass to his/her parents.
Therefore, you might think that in the event of either of you dying your assets will pass to each other, and if the other spouse is not alive then it will pass to your parents (not parents-in-law).
Let’s test this scenario. Let’s say both of you die simultaneously in a car accident and the state you live in applies the rule that the older person dies first and does not require the younger person to pass a survival period.