When two people die at the same time i.e. simultaneous death, the 'commorientes' rule of the Property Act 1925 applies i.e. the older person is deemed to have died first.
Under our Commorientes Rule 2 for death at same time, when calculating IHT on the estate of the younger spouse/civil partner, the additional assets (technically inherited from older 1st death spouse are ignored. This is due to at that instantaneous point of joint death - the younger spouse/partner never actually received the wealth from partner, so ignored completely from 2nd death IHT bills.
However, and we will call it under Commorientes Rule 3, if your Will has a 28 day survival clause for all beneficiaries then a simultaneous death would mean that on 1st death (older spouse), the 2nd younger spouse would never have inherited your wealth anyway and it automatically passes to your next line of beneficiaries meaning that on 1st death (older spouse), the usual inheritance tax allowances will be used and then IHT may be payable on both the 1st death (older spouse) estate and also payable on the 2nd death (younger spouse) estate if those separate estates are valued individually over their respective IHT allowances.