How to handle multiple Oracle homes?

Assuming that you don't want multiple tnsnames.ora files, the simplest option would be to set the TNS_ADMIN environment variable to point at the one tnsnames.ora file you wanted to maintain on the machine. All the Oracle Homes on the machine would then use the tnsnames.ora file in the directory specified by the TNS_ADMIN setting before looking for an entry in the local Oracle Home's tnsnames.ora file.

On windows Oracle Locator Express is a simple (and free) tool to handle setting the oracle home and/or the TNS_ADMIN environment variable.