gistfile1.py

from django.contrib.auth.models import User as AuthUser
from django.db.models.signals import post_save, post_delete

class BaseUser(AuthUser):
    class Meta:
        abstract = True

    @classmethod
    def __sync__(cls, from_user, to_user):
        if not from_user.pk and to_user.pk:
            to_user.delete()
            return to_user

        from_fields = from_user._meta.get_all_field_names()
        to_fields = to_user._meta.get_all_field_names()

        for field in set(from_fields) & set(to_fields):
            setattr(to_user, field, getattr(from_user, field))

        to_user.save()
        return to_user

class User(BaseUser):
    avatar = models.ImageField(upload_to='avatars/')

def sync_user(sender, instance, **kwargs):
    other = AuthUser if sender == User else User
    user = User.__sync__(instance, other(pk=instance.pk))

post_save.connect(sync_user, sender=AuthUser)
post_save.connect(sync_user, sender=User)
post_delete.connect(sync_user, sender=AuthUser)
post_delete.connect(sync_user, sender=User)