When calculating joint mortality, Voyant will normally assume simultaneous death and that the estate of the eldest client passes to the estate of the younger, inclusive of the Nil Rate Band transfer, before being taxed and then distributed. This assumption is made provided that alternative estate distribution arrangements have not been entered on the Estate Plans screen.
A special setting is available, however, for cases where clients have a predeceased clause in their wills or if they live in a jurisdiction where this is assumed. You will find this setting in Dashboard > Plan Settings > Calculation Settings > Assume predeceased upon simultaneous death
This setting might be better named the 'Simultaneous Death Clause'. The essential meaning of the clause (which would be entered into the will for each individual) is as follows:
If my spouse and I shall die under such circumstances that the order of our deaths cannot be readily ascertained, my spouse shall be deemed to have predeceased me. No person, other than my spouse, shall be deemed to have survived me if such person dies within (e.g.) 30 days after my death.
This clause helps avoid the sometimes time-consuming problems that occur if both spouses die together in an accident. Your spouse's will should contain an identical clause; even though it seems contradictory to have two wills each directing that the other spouse died first, since each will is probated by itself, this allows the estate plan set up in each will to go forward as you planned. The second sentence exists to prevent the awkward legal complications that can ensue if someone dies between the time you die and the time the estate is divided up. Instead of passing through two probate processes, your gift to a beneficiary who dies shortly after you do would go to whomever you would have wanted it to go had the intended beneficiary died before you did. Most such gifts go into the residuary estate.
It is a setting that the software applies on the assumption of simultaneous mortality, and it might be appropriate, for example, where each spouse has children from a previous relationship, who they want to ensure receive the benefits from their respective estates.
However, in most instances, it is liable to give you outputs that you are not expecting so it is best toggled off by default.