Simultaneous death rules mainly affect married couples and registered civil partners. Remember, the oldest is deemed to die first.
No inheritance tax is usually payable when wealth is transferred on death between spouses anyway. It may then only be payable with all accumulated wealth on 2nd death.
However, under our Commorientes Rule 2, for death at the 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 because at the instantaneous point of joint death - the younger spouse/partner never actually received the wealth from 1st older spouse, so it is ignored completely in 2nd death IHT bills.