For each applicant, in the field "matrimonialPropertyScheme" in the object "applicantDetail", we can send information how the property is owned:
jointEstate
separateEstate
contributionToJointAcquiredProperty

In Case, the mortgage is for two debtor, but the property is owned by only Debtor A of these two debtor:
What do you send in this field for Debtor A? --> I would say "separateEstate"
What do you send in this field for Debtor B? --> There is no enum for this. Do you send it empty?

I would suggest to add a new enum: "none" or "n-a". So that we can send a clear message to the FI.