How to create oracle user for more than one version
For a start you can use oraenv script to set PATH, LD_LIBRARY_PATH, ORACLE_BASE and ORACLE_HOME:

export ORACLE_SID=ORCL

ORAENV_ASK=NO
. /usr/local/bin/oraenv


ORACLE_SID can be parsed from /etc/oratab file. ORACLE_HOSTNAME maybe from HOSTNAME environment variable.