models.py:

from django.db import models


class ChildModel(models.Model):
    name = models.CharField(max_length=200)


class ParentModel(models.Model):
    name = models.CharField(max_length=200)
    child_1 = models.ForeignKey(ChildModel, on_delete=models.SET_NULL,
                                related_name='parents_1', null=True)
    child_2 = models.ForeignKey(ChildModel, on_delete=models.SET_NULL,
                                related_name='parents_2', null=True)

Django shell session:

from testapp.models import ParentModel, ChildModel

child_1 = ChildModel.objects.create(name="child_1")
child_2 = ChildModel.objects.create(name="child_2")
parent_1 = ParentModel.objects.create(name="parent 1", child_1=child_1, child_2=child_2)
parent_2 = ParentModel.objects.create(name="parent 2", child_1=child_2, child_2=child_1)

child_1.delete()
parent_1 = ParentModel.objects.get(pk=parent_1.pk)
parent_2 = ParentModel.objects.get(pk=parent_2.pk)
# parent_1.child_2 and parent_2.child_1 should be normaly equal to child_2 but...
parent_1.child_2 is not None and parent_2.child_1 is not None
# False is returned