fields.py

from django.db import models

class DependentCharField(models.CharField):
  description = "A Field that can be null or blank if other returns a truthy value"

  def __init__(self, dependentName, *args, **kwargs):
    kwargs['blank'] = True
    kwargs['null'] = True

    self.dependentName = dependentName
    return super(DependentCharField, self).__init__(*args, **kwargs)

  def pre_save(self, model_instance, add):
    otherValue = getattr(model_instance, self.dependentName, None)
    thisValue = getattr(model_instance, self.attname, '')              
        
    if otherValue or thisValue:
      return super(DependentCharField, self).pre_save(model_instance, add)
    raise models.exceptions.ValidationError("%s can't be blank or null if %s is" % (self.attname, self.dependentName))

gistfile2.txt

In [2]: t = TestModel()

In [3]: t.save()

ValidationError: [u"test can't be blank or null if name is"]

In [4]: t = TestModel(name="foo")

In [5]: t.save()

In [6]: t = TestModel(test="bar")

In [7]: t.save()

models.py

from django.db import models
from fields import DependentCharField

class TestModel(models.Model):
    name = models.CharField(max_length=25)
    test = DependentCharField('name', max_length=25)