1. You can have one value for each variable in your profile file.e.g.
If you have the following parameters in your profile file:

ORACLE_HOME=/export/oracle/app/product/8.1.5

ORACLE_SID=HOST

And, you want to start CODE database, you need to change the ORACLE_HOME and ORACLE_SID at $prompt.

2. oratab file parameters:

HOST:/export/oracle/app/product/8.1.5:N
CODE:/export/oracle/app/product/8.1.6:N

This is for starting the database at system bootup.

3. For listener.ora: I guess you need to add one more ADDRESS for the new software i.e. oracle 8.1.6.0 with change SID_LIST_LISTENER that you did already.

Regards,

abhijit