Because you’re older, you’re deemed to have died first, so your will is distributed first. Therefore, all of your assets pass to your spouse. Your spouse’s will is now exercised, and as you’re not alive, your spouse’s assets pass to his/her parents (your parents-in-law).
This would result in your parents-in-law receiving both yours and your spouse’s assets (including both businesses) and your parents miss out!
One way to potentially resolve this situation is to ensure your spouse’s will states that if he/she owns any shares in your business that on his/her death these be left to your parents (and vice versa).
This is just one example that highlights how a simple estate plan can be more complex than first expected. For every situation there are many different scenarios. Good legal advice is important.