This situation is covered by the “IHT Payment Record” function in Probate Plus.
Rather than entering the IHT payable as one or more administration expenses, you simply record the total IHT due in the IHT Payment Record and then allocate the share due by each of the non exempt beneficiaries as entries in the IHT Payment Record.
The IHT Payment Record is found under menu “Activities..IHT Payment Record”. From here, set the IHT amount and add your IHT allocations to the beneficiaries. The amount of IHT allocated will be shown on the distribution summary report for the each beneficiary in question.
Probate Plus online help also covers this under book 4 “Using Probate Plus..Admin tasks..Administering the estate..Paying IHT”.