There is the doctrine of mutual wills though - this is where there is a contract between two parties not to alter or change the wills. These are often confused with mirror image wills but are quite different animals.
However, there has to be good evidence of a contract - the mirroring of the wills is not enough. Even then it can still be changed, although whether the court will sanction those changes is a different matter. If such a contract to make wills is found, the survivor will hold their estate on constructive trust for the beneficiaries under the mutual wills. However, this is an extremely technical and rare area of the law and if in doubt you should take advice. The likely scenario though is that it is a H&W and they made mirror image wills.