How to handle error messages when validating a form across different FieldLists of FormFields?

from collections import defaultdict

class AllocationForm(Form):
    project_id = StringField('Project ID')
    # ...

class DeallocationForm(Form):
    project_id = StringField('Project ID')
    # ...

class RequestForm(FlaskForm):
    allocations = FieldList(FormField(AllocationForm))
    deallocations = FieldList(FormField(DeallocationForm))

    def validate(self):
        data_fields = defaultdict(list)
        for field in (self.allocations, self.deallocations):
            for form in field:
                if form.project_id.data:
                    data_fields[form.project_id.data].append(form.project_id)

        success = True
        for data,fields in data_fields.items():
            if len(fields) > 1:
                for field in fields:
                    msg = 'A project ID cannot appear more than once in a request'
                    field.errors += (msg,)
                success = False

        return success and super().validate()