class A(models.Model):
    ....
    class Meta:
        constraints = [
            UniqueConstraint(
                name="unique_one",
                fields=['field_a', 'field_b'],
                deferrable=Deferrable.IMMEDIATE
            ),
            UniqueConstraint(
                name='unique_two',
                fields=['field_a', 'sort_index'],
                deferrable=Deferrable.DEFERRED
            )
        ]

class ASerializer(serializers.ModelSerializer):
    class Meta:
        model = A
        fields = '__all__'
        validators = [
            UniqueTogetherValidator(
                queryset=A.objects.all(),
                fields=['field_a', 'field_b']
            )
        ]


class B(models.Model):
    ...
    class Meta:
        constraints = [
            UniqueConstraint(
                name='unique_one',
                fields=['field_a', 'sort_index'],
                deferrable=Deferrable.DEFERRED,
            )
        ]

class BSerializer(serializers.ModelSerializer):
    class Meta:
        model = B
        fields = '__all__'

list_of_a = [
    B(name="A", field_a=1, sort_index=1),
    B(name="B", field_a=1, sort_index=2)
]
b_serializer = BSerializer(data={"name":"C", "sort_index":1})
b_serializer.save()

# final list of Bs
list_of_b = [
    B(name="C", field_a=1, sort_index=1),
    B(name="A", field_a=1, sort_index=2),
    B(name="B", field_a=1, sort_index=3)
]