Oracle db_name, db_unique_name, and ORACLE_HOME
You can have 2 databases with the same db_name on the same system on linux in the same ORACLE_HOME. You just have to make sure that the instance name differs. EG different pfile/spfile,... It's just not advised to give them the same name...
When running the runinstaller you just give them the same global name and a different SID. 
--> Not sure how Windows reacts on it.
The reason why the db_name has to be the same in a primary-standby setup is because the name is "imprinted" on the datafiles. The db_unique_name is mandatory in order to make a difference between the databases and to setup a syncing. The recovery would be impossible if you use a different db_name