I'm not sure I follow here; this seems like something on the Manila side not Cinder or Nova. If I just create two volume (or 3 or 4) in Cinder and attach them to an instance that seems to work just fine:

jgriffith@bdr75:~/devstack$ nova volume-attach 15fb3379-1811-46ca-bdc3-35cccafb1233 1bf11fab-2329-4506-9cdb-3bf4af8688bc
+----------+--------------------------------------+
| Property | Value                                |
+----------+--------------------------------------+
| device   | /dev/vdb                             |
| id       | 1bf11fab-2329-4506-9cdb-3bf4af8688bc |
| serverId | 15fb3379-1811-46ca-bdc3-35cccafb1233 |
| volumeId | 1bf11fab-2329-4506-9cdb-3bf4af8688bc |
+----------+--------------------------------------+
jgriffith@bdr75:~/devstack$ nova volume-attach 15fb3379-1811-46ca-bdc3-35cccafb1233 51dbb65b-5da8-435c-ae2d-895efac90dca

+----------+--------------------------------------+
| Property | Value                                |
+----------+--------------------------------------+
| device   | /dev/vdc                             |
| id       | 51dbb65b-5da8-435c-ae2d-895efac90dca |
| serverId | 15fb3379-1811-46ca-bdc3-35cccafb1233 |
| volumeId | 51dbb65b-5da8-435c-ae2d-895efac90dca |
+----------+--------------------------------------+
jgriffith@bdr75:~/devstack$
jgriffith@bdr75:~/devstack$ nova volume-attach 15fb3379-1811-46ca-bdc3-35cccafb1233 a7c1c1eb-246b-4531-95fb-d6068ef3313e
+----------+--------------------------------------+
| Property | Value                                |
+----------+--------------------------------------+
| device   | /dev/vdd                             |
| id       | a7c1c1eb-246b-4531-95fb-d6068ef3313e |
| serverId | 15fb3379-1811-46ca-bdc3-35cccafb1233 |
| volumeId | a7c1c1eb-246b-4531-95fb-d6068ef3313e |
+----------+--------------------------------------+
jgriffith@bdr75:~/devstack$ nova list; cinder list
+--------------------------------------+------+--------+------------+-------------+---------------------+
| ID                                   | Name | Status | Task State | Power State | Networks            |
+--------------------------------------+------+--------+------------+-------------+---------------------+
| 15fb3379-1811-46ca-bdc3-35cccafb1233 | test | ACTIVE | -          | Running     | private=192.168.0.5 |
+--------------------------------------+------+--------+------------+-------------+---------------------+
+--------------------------------------+--------+----------+------+-------------+----------+--------------------------------------+
| ID                                   | Status | Name     | Size | Volume Type | Bootable | Attached to                          |
+--------------------------------------+--------+----------+------+-------------+----------+--------------------------------------+
| 1bf11fab-2329-4506-9cdb-3bf4af8688bc | in-use | -        | 1    | lvmdriver-1 | false    | 15fb3379-1811-46ca-bdc3-35cccafb1233 |
| 51dbb65b-5da8-435c-ae2d-895efac90dca | in-use | -        | 1    | lvmdriver-1 | false    | 15fb3379-1811-46ca-bdc3-35cccafb1233 |
| 6d4107b1-78a4-45e1-92c2-05182f4aab38 | in-use | fedora24 | 10   | solidfire   | true     |                                      |
| a7c1c1eb-246b-4531-95fb-d6068ef3313e | in-use | -        | 1    | lvmdriver-1 | false    | 15fb3379-1811-46ca-bdc3-35cccafb1233 |
| d8e6dda2-2f00-4db7-a101-fea3ad5f011a | in-use | cirros   | 1    | solidfire   | true     |                                      |
+--------------------------------------+--------+----------+------+-------------+----------+--------------------------------------+
jgriffith@bdr75:~/devstack$


``````````````````

ssh onto the Instance and:
$ ls /dev | grep vd
vda
vdb
vdc
vdd
$

Our volume sare vdb, vdc, and vdd