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][1] (emphasis mine): 

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

Also, [the installation guide][2] 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.


  [1]: https://docs.oracle.com/database/121/LADBI/inst_task.htm#LADBI7742
  [2]: https://docs.oracle.com/database/121/LADBI/usr_grps.htm#LADBI7678