I have previously explained the difference between Mirror Wills and Mutual Wills. If a couple intend that their Wills be Mutual Wills (ie that the survivor will not change his or her Will), then the safest way to document the agreement is to have a Contract between the parties (entered into at the time when the Wills are signed). The Contract is relatively simple and outlines the agreement between the two Will makers. It provides that neither party can change his or her Will without the agreement of the other party. It follows that after the death of the first of the couple, such consent cannot be obtained and therefore the Will of the survivor cannot be changed. If there is a breach of the agreement, then the usual remedies are available to those parties (ie beneficiaries) who may have been prejudiced or suffered a loss as a result of the breach. Another way of documenting the agreement is for each party to make a statutory declaration at the time of making the Will to the effect that an agreement exists between the two parties to the effect that the Wills shall not be changed after the first dies. In effect, the statutory declarations are evidence that Mutual Wills exist and there is an agreement between the parties however, it is not the actual agreement.
Another way to protect the beneficiaries under the Mutual Wills is to incorporate a clause into the Wills to the effect that there is an agreement between the couple that the survivor shall not change the Will. The Will contract also usually provides that the survivor will not diminish the assets except as is necessary to pay for the reasonable living and day to day expenses of the survivor.
Mutual Wills is a complex area of the law but can be very important, particularly in the event of making fair provision for beneficiaries in blended families.