def save(self, *args,  **kwargs):
    user_id = self.id
    super(User, self).save(*args, **kwargs)
    if user_id is None:
        as_A = A(user=self)
        as_A.save()
        as_B = B(user=self)
        as_B.save()

class A(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    x = models.CharField(max_length=100, blank=True)

class B(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    y = models.CharField(max_length=100, blank=True)

class RegistrationForm(forms.ModelForm):
    password = forms.CharField(
        strip=False,
        widget=forms.PasswordInput,
    )
    confirm_password = forms.CharField(
        strip=False,
        widget=forms.PasswordInput
    )

    class Meta:
        model = User
        fields = ['email', 'first_name', 'password']

    def clean(self):
        cleaned_data = super(RegistrationForm, self).clean()
        password = cleaned_data.get("password")
        confirm_password = cleaned_data.get("confirm_password")
        if password != confirm_password:
            raise forms.ValidationError("Passwords are not same")
        return cleaned_data