AFAIK you can't configure nova to connect to multiple backends, but cinder can. You can create several of these entries in the cinder.conf and choose during volume creation which backend to choose.

[rbd]
volume_driver = cinder.volume.drivers.rbd.RBDDriver
rbd_pool = pool1
rbd_user = user1
rbd_ceph_conf = /etc/ceph/ceph.conf
rbd_secret_uuid = <SECRET
volume_backend_name = rbd

[rbd2]
volume_driver = cinder.volume.drivers.rbd.RBDDriver
rbd_pool = pool2
rbd_user = user2
rbd_ceph_conf = /etc/ceph1/ceph.conf
rbd_secret_uuid = <SECRET>
volume_backend_name = rbd2

Then also set this:

enabled_backends = rbd,rbd2

And via Horizon or the OpenStack CLI you can create a new volume type that connects to the desired backend:

control:~ # openstack volume type show ssd
+--------------------+--------------------------------------+
| Field              | Value                                |
+--------------------+--------------------------------------+
| access_project_ids | None                                 |
| description        | None                                 |
| id                 | 152cb8cb-0f64-4064-9235-8138418f73b9 |
| is_public          | True                                 |
| name               | ssd                                  |
| properties         | volume_backend_name='rbd2'           |
| qos_specs_id       | None                                 |
+--------------------+--------------------------------------+

I believe it's supported as mentioned here;

https://medium.com/walmartlabs/deploying-cinder-with-multiple-ceph-cluster-backends-2cd90d64b10 (https://medium.com/walmartlabs/deploy...)

and here;

https://docs.openstack.org/openstack-ansible-ceph_client/latest/configure-ceph.html#extra-client-configuration-files (https://docs.openstack.org/openstack-...)

http://www.sebastien-han.fr/blog/2015/09/15/openstack-nova-configure-multiple-ceph-backend-on-one-hypervisor/ (http://www.sebastien-han.fr/blog/2015...)