Oracle 10 and Oracle 11 clients coexistence

As long as you install the two versions of the Oracle client in different Oracle Homes, they should coexist peacefully. There are just a couple of gotchas to be aware of

1. By default, each Oracle Home will have a separate tnsnames.ora file (and sqlnet.ora file, etc.). That often causes confusion if you've configured a TNS alias in one Oracle Home and not in the other. You can configure your environment to use a single set of TNS configuration files by setting the TNS_ADMIN environment variable to point at the directory that contains the one true source of TNS configuration issues (i.e. set TNS_ADMIN to %Oracle11g_Home%\network\admin to always use the tnsnames.ora file from your 11g Oracle Home).
2. Some third-party products are not multi-home compliant. If you use something like the legacy Microsoft ODBC Driver for Oracle, for example, it will use whichever version of the Oracle client appears first in your PATH. If you are using Oracle drivers to connect to the database, that shouldn't be an issue. If you do need to switch which is the default Oracle Home, you can either manually edit your PATH or you can fire up the Oracle Universal Installer and under Installed Products | Environment, you can control the order that Oracle Homes appear in the PATH.