How to create Oracle RAC database using dbca silent mode with different db_name and db_unique_name
Since you asked RAC, please see if you can use this non-RAC example as a starting point.
For db_name != db_unique_name != sid
- Non-RAC
- Oracle 12.1
- Custom Database (not seed)

On the command line, set ddbname == parameter db_unique_name, and also set parameter db_name

dbca    \
-silent \
-createDatabase \
-templateName    wailua.dbt     \
-sid wailua20           \
-gdbname wailuaq        \
-initParams db_unique_name=wailuaq,db_name=wailua       \


where wailua.dbt is copied from New_Database.dbt

db_name and db_unique_name are not mentioned in wailua.dbt

Note that variables 
SID and DB_UNIQUE_NAME expand in the template.

[oracle@waipio1 wailua20 templates]$ echo $ORACLE_SID
wailua20

[oracle@waipio1 wailua20 templates]$ sqlplus / as sysdba

SQL*Plus: Release 12.1.0.1.0 Production on Wed Jun 11 00:48:22 2014

Copyright (c) 1982, 2013, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
With the Partitioning, Automatic Storage Management, OLAP, Advanced Analytics
and Real Application Testing options

SQL&gt; show parameter db_name

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_name                              string      wailua
SQL&gt; show parameter db_unique_name

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_unique_name                       string      wailuaq

oratab:
wailua20:/app/oracle/product/12.1.0/db_1:N:         # line added by Agent

ASM:
asmcmd ls data/WAILUAQ/
CONTROLFILE/
DATAFILE/
ONLINELOG/
PARAMETERFILE/
TEMPFILE/
spfilewailua20.ora


Full crdb.wailua.ksh dbca script: http://pastebin.com/vx3k5g3U/ 

dbca log: http://pastebin.com/8UQDEDZn/