Django runserver custom database

I'm using Django 1.4

I need to start the development server and I want to specify (in the command) which database it must use. For example if my settings contains:

DATABASES = {
    'default': {
        'NAME': 'db.db',
        'ENGINE': 'django.db.backends.sqlite3'
    },
    'tests': {
        'NAME': 'tests.db',
        'ENGINE': 'django.db.backends.sqlite3'
    },
}

I want to use something like:

django-admin.py runserver --database=tests

There is something similar? I also tried to use a second setting file but the --settings option doesn't work: (--settings option seems to be deprecated, in DOCS there is no mention at all)

django-admin.py runserver --settings=settings_tests

or

django-admin.py runserver --settings settings_tests

raises an error:

ImportError: Could not import settings 'settings_tests' (Is it on sys.path?): No module named settings_tests