You seem to have a mix of Divorce legal ownership rights and Estate/tax issues here and it isn't clear why you are asking. If the beneficiary of a retirement account dies before that provision is triggered, there is no ownership now of the spouse's accounts, unless the spouse also dies without changing the beneficiary. The retirement account of the spouse would not be distributable as part of the deceased person's estate, for instance. And in some States, retirement accounts are protected from legal actions.

I recommend asking a lawyer in your region.