Make the document fit your circumstances
A single will records one person’s wishes. Mirror wills are separate wills with similar provisions, often considered by couples; they are not one joint document. Each person’s wishes and circumstances still need consideration. Blended families, overseas assets, business interests or a possible claim against the estate can make specialist advice important.
Writing the words is only part of making a will. It must be signed and witnessed correctly under the law that applies. Keep the completed document safely and let your executors know where it is. Review it when your family or financial circumstances change. The general guidance here concerns England and Wales; arrangements elsewhere need advice for that jurisdiction.