It is fairly common in estate planning attorneys' offices, for a husband and wife to come in and declare that they both want similar wills drawn up. These wills are often referred to as "mirror image wills."
The most common form they take, is that each spouse gets a will leaving everything he or she owns to the surviving spouse. The second to pass away spouse, then gives everything to the children or other agreed upon heirs.
Despite their seeming simplicity, these wills are an unusually common source of litigation, as the National Law Review discusses in "Contracts to Make Wills or Trusts."
The problem starts when the surviving spouse has a change of plans and changes his or her will to divide things differently or to give the estate to different heirs.
The heirs of the original mirror image wills routinely argue in court, that the spouses entered into a contract to make the original wills. Unfortunately, that is simply not the case in almost all circumstances.
To be valid, a contract requires that a person receive some sort of compensation, called consideration, for whatever promise it is that they are contracted to perform.
In the case of mirror image wills, spouses rarely receive any form of consideration for promising not to change the will later.
It is important to understand this point, because the issue frequently comes up in estate litigation. It costs estates a lot of money, when the issue is raised.
Reference: National Law Review (April 10, 2017) "Contracts to Make Wills or Trusts."