Oracle 12c installation
Docs don't just "suggest" that you "can" install into a new location.  It is required.  Two different installations must be in their own ORACLE_HOME directories.  And you do not "add" another path/directory to ORACLE_HOME. ORACLE_HOME specifies only one home/directory at a time.

To quote from the Installation Guide (emphasis mine): 


  "Do not install Oracle Database 12c software into an existing Oracle
  home."


Also, the installation guide specifically says to unset ORACLE_HOME and several other environment variables before launching the OUI.


  If you have had an existing installation on your system, and you are
  using the same user account to install this installation, then unset
  the ORACLE_HOME, ORACLE_BASE, ORACLE_SID, TNS_ADMIN environment
  variables and any other environment variable set for the Oracle
  installation user that is connected with Oracle software homes.