Provided you're capable, your will is generally revocable-you can change or cancel it as you see fit but, as is often the case in law, you can agree otherwise.
Many couples visit lawyers together to prepare wills. The result is often mirror wills (a.k.a. joint or reciprocal wills): two wills that are nearly identical to one another. For example, each spouse or partner may leave almost everything to the survivor between them and select an alternative beneficiary in the event the other spouse or partner dies first.
Mutual wills are less common. With mutual wills, two people, usually (not necessarily) spouses or partners, enter into an agreement not to revoke their wills thereby locking in the distribution of property in a certain way. The advantage is that the surviving spouse or partner cannot change his or her will to distribute the property in a manner not agreed to by the deceased spouse or partner.
On the first death, the deceased has met his or her end of the bargain by disposing of property under the mutual will and the agreement. The expectation and promise of the survivor is that he or she will do the same. If not, remedies are available in the form of damages and constructive trusts (i.e., that the property is held in trust for the benefit of the person agreed upon between the spouses or partners).
The agreed upon distribution may be for property generally or for specific items. For example, a couple might agree to give the survivor the benefit of a cottage or shares for survivors lifetime and on the death of the survivor to give those items to certain individuals. The agreement may also address the extent to which the survivor is able to use or dispose of property in his or her lifetime.
Importantly, mirror wills are not mutual wills. Regardless of the names applied, the fact that a couple makes nearly identical wills at the same time will likely not be sufficient to bind that couple to a certain distribution scheme and prevent either of them from revoking.
Mutual wills must be established by very clear evidence outside the wills themselves. Careful review and drafting by a lawyer is needed. The best evidence will be a written agreement; however, there will be circumstances where inferences may be made from documents from the preparation of the wills, conduct of the parties and other evidence that goes to the intention of the parties.
Consult with a lawyer about the roll of mirror and mutual wills in the context of your estate planning objectives.