Simply put, a Mutual Will is an agreement between two individuals (minimum) to make their Wills at the same time on agreed terms. These terms are usually captured in a single document signed by both parties. The document becomes legally binding and is a contractual agreement between the parties.
When one party to the Mutual Will dies, the survivor is unable to change the terms of the Will. If the survivor breaches the agreement and put in place a new document contrary to the previously agreed terms, then the beneficiaries of the Mutual Will, will have the right to bring a claim for breach of trust to ensure the terms of the original agreement are upheld.
This can become very complicated, and it can be difficult to ascertain whether a Mutual Will is in place if this is not disclosed or are not fully understood by the survivor to be binding.
Where both parties wish to change the Mutual Will and have the capacity to do so, then new Wills can be put in place.