[Answer]-How can I save an empty model (I only need it's pk created)

“Storage” that holds 2 “StorageType”

That means Storage can have many StorageType and StorageType can have Storages. Make a new ManyToMany Field in the storage.

class Storage(models.Model):
    storage_types = models.ManyToManyField(StorageType)

    def __unicode__(self):
        return str(self.id)

class StorageType(models.Model):
    tripa = 'Tripa'
    portada = 'Portada'
    type_choice = (
        (tripa, 'Tripa'),
        (portada, 'Portada'),
    )
    sto_type = models.CharField(max_length=9, choices=type_choice, default=tripa)
    paper_type = models.ForeignKey(Paper)
    paper_qnty = models.IntegerField(blank=True, default=0)
    web_paper_qnty = models.IntegerField(blank=True, default=0)

    def __unicode__(self):
        return '%s of %s' %(self.sto_type, str(self.storage))

Then read Django Many-to-many relationships