If you are intent on installing two copies of the software, each in its own ORACLE_HOME, you may as well "trick out" the installer by making separate copies of what's in /var/opt/oracle (the oraInst.loc file). Run OUI, and it thinks there is only the one version of Oracle installed. Do whatever you need to do to the installed files. When it's time to work on the other installed version, switch the loc file.