People who have substantial assets in more than one nation, might need more than one will to have an effective estate plan.
Ordinarily people only have one will. They cannot have more than one. If they create a second will, then the first will is no longer valid.
The last will a person drew up and executed, is the only will that should be used in the absence of extraordinary circumstances to settle an estate.
However, it is not always technically true.
There are people who might need more than one will. If you have assets in more than one country and are a citizen of both countries, then you might need a valid will in each country, as the Financial Review explains in "Double trouble for dual nationals."
The problem is that some countries have strict laws about who can inherit certain property. There are laws about how much of an estate must be given to a spouse and to children.
Most countries do not allow deviation from these laws, even for people who do not live there full time and who have a valid will in another nation.
Even if your will is valid in the U.S., it is possible that another country where you hold assets could invalidate it for the property you hold in that country.
If your estate might be subject to the laws of more than one nation, make sure that your estate plan is valid in all the nations where you own property. If that does not seem possible, then have separate estate plans for the property in each nation.
Reference: Financial Review (Sep. 20, 2017) "Double trouble for dual nationals."