Could someone please tell me how to set up the IFILE if I have two Oracle on the box (8.1.6 and 8.1.7)

this is the case: If I have two home say
c:\oracle\ora816\admin\DB1\pfile\initDB1.ora

After I upgrade this DB1 database to 8.1.7, how do I set up the pfile pointed to the old initDB1.ora by using IFILE ???


and my new home c:\oracle\ora817\admin\

Where do I put the IFILE at ??? and how to set it up to point to the old init.ora file

Thanks