Assuming no IHT on the first death the deceased’s NRB will remain in tact as will the RNRB. Or maybe some part of the NRB will be used depending upon to what extent the taxable estate exceeds the 50% settled on the IPDI.
On the survivor’s death the survivor’s estate comprises the 50% interest in the house held absolutely (plus any other assets in the free estate) plus the value of the IPDI.
The survivor’s estate is reduced by possibly two NRB’s plus two RNRBs. The net estate (after IHT, if any) is then apportioned between the survivor’s free estate and the trustees of the IPDI interest.
These two apportioned amounts of IHT will be equal if the survivors free estate equals in the value of the IPDI interest. Neither of the spouses beneficiaries will then be worse off then the other.
However, an unequal allocation of IHT only arises if the surviving spouse’s free estate is greater than the value of the IPDI; eg free estate 350k and IPDI 150k. In this case the free estate bears 350k/500k (70%) and the trustees of the IPDI (150k/500k) 30%.
The NRBs and the RNRBs are simply deducted from the the aggregate (free plus IPDI) taxable estate of there survivor and cannot be allocated separately against the free estate and the IPDI. The RNRBs are deducted first.