Environment variable for two oracle_home

As @ik_zelf mentioned, your PATH variable will need to include the "bin" directory of your Oracle software install. This will allow you to execute commonly-needed commands/utilities like "sqlplus" and "lsnrctl" without changing directories. This is typically $ORACLE_HOME/bin

Your TNS_ADMIN variable needs to be set in a similar matter, pointing to the location of your tnsnames.ora, listener.ora, etc. By default, SQL Plus checks this directory (defined by TNS_ADMIN) for tnsnames.ora. This is typically $ORACLE_HOME/network/admin

If you want to connect to your database via TNS, your tnsnames.ora file (assuming a ORACLE_SID of "myDatabase", a host of "myHostname" and the default port) would look something like this:

myDatabase = 
    (DESCRIPTION = 
        (ADDRESS = (PROTOCOL = TCP)(HOST = myHostname)(PORT = 1521))
        (CONNECT_DATA = (SERVICE_NAME = myDatabase)))

This would allow you to connect locally simply by doing the following from a command line:

sqlplus username/password@myDatabase

Of course, as SQL Plus looks for the tnsnames.ora file in the location defined by $TNS_ADMIN, this will only function locally. To access this database via TNS (Transparent Network Substrate) from a remote machine, you would have to install the Oracle client and ensure that the TNS entry for myDatabase (as described above) is included in the remote machine's tnsnames.ora file as well.

First of all, I should thank you for all other steps that you mentioned above. As an addition to your points, I have this one which I've been using so far.

Solaris and Linux

$ pgrep  -lf _pmon_
12586 ora_pmon_ee10204

ORACLE_SID is ee10204

$ pwdx 12586
12586: /u01/oracle/10.2.0.4/ee1/dbs

ORACLE_HOME is /u01/oracle/10.2.0.4/ee1

HPUX

$ ps -ef | grep pmon
ora1024 25611     1  0  Mar 21  ?         0:24 ora_pmon_itanic10

ORACLE_SID is itanic10

$ pfiles 25611 | grep  bin
25611:                  /opt/ora1024/app/db_1/bin/oracle

ORACLE_HOME is /opt/ora1024/app/db_1

AIX

$ ps -ef | grep pmon
ora1024   262314        1   0   Mar 23      -  0:12 ora_pmon_mercury

ORACLE_SID is mercury

$ ls -l /proc/262314/cwd
lr-x------   2 ora1024  dba  0 Mar 23 19:31 cwd -> /data/opt/app/product/10.2.0.4/db_1/dbs/

ORACLE_HOME is /data/opt/app/product/10.2.0.4/db_1

Personally, I haven't seen any environment that tries to confuses the person who will maintain the system. Most of the time, I've always seen the normal /u01/app/oracle for the ORACLE_BASE.