[Django]-Howto combine DjangoRestFramework routers for different apps

I simply achieved this by extending the router registries.

root urls.py

from ad.urls import router as ad_router

main_router = routers.DefaultRouter()
main_router.registry.extend(ad_router.registry)

ad.urls.py

from .api.urls import router as api_router

router = routers.DefaultRouter()
router.registry.extend(api_router.registry)

ad.api.urls.py

router = routers.DefaultRouter()
router.register(r'ad', AdViewSet)

In your main urls.py you would do something like this.
(This is for Django 1.8)

urlpatterns += patterns('',
url(r'^api/', include(patterns('',
    url(r'^foo_app/', include('foo_app.urls')),
    url(r'^bar_app/', include('bar_app.urls')),
    url(r'^test_app/', include('test_app.urls')),
))))

this allows you to access all your endpoints this way

localhost:8000/api/foo_app/<foo_app_endpoint>
localhost:8000/api/bar_app/<bar_app_endpoint>
localhost:8000/api/bar_app/<test_app_endpoint>