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 directory. 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: (https://docs.oracle.com/database/121/LADBI/inst_task.htm#LADBI7742):

"Do not install Oracle Database 12c software into an existing Oracle home." (emphasis mine).

Also, the installation guide specifically says to unset ORACLE_HOME and several other enviornment 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." https://docs.oracle.com/database/121/LADBI/usr_grps.htm#LADBI7678

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.