How to record log info to flat file + Event Log

I have a 3rd-party app that logs (verbosely) to the users' Appdata folder. I'd like to augment that behavior to also record Errors to the EventLog. How can I accomplish this? Note that the app's source code is not available; I can only modify the .config file.

Here's the original .config file. It logs verbosely to a .log file:

<system.diagnostics>
    <switches>
        <add name="Default" value="Verbose"/>
    </switches>
    <trace autoflush="true" indentsize="4">
        <listeners>
            <add name="CPLog"
                type="ClientApp.Common.Logging.MultiProcessFileTraceListener, ClientApp.Common, Version=9.1.0.0, Culture=neutral, PublicKeyToken=c583c5a4dddb9a96"
                initializeData="@(LocalApplicationData)\ClientApp\cpwin.log" />
            <remove name="Default" />
        </listeners>
    </trace>
</system.diagnostics>

Here's what I tried (added another switch and another listener. EventLog gets written if I set its switch to Verbose, but nothing gets ever written when set to Error (even when errors occur):

<system.diagnostics>
    <switches>
        <add name="Default" value="Verbose"/>
        <add name="EventLog" value="Error"/>
    </switches>
    <trace autoflush="true" indentsize="4">
        <listeners>
            <add name="CPLog"
                type="ClientApp.Common.Logging.MultiProcessFileTraceListener, ClientApp.Common, Version=9.1.0.0, Culture=neutral, PublicKeyToken=c583c5a4dddb9a96"
                initializeData="@(LocalApplicationData)\ClientApp\cpwin.log" />
            <add name="EventLog" type="System.Diagnostics.EventLogTraceListener" initializeData="Client Info" />
            <remove name="Default" />
        </listeners>
    </trace>
</system.diagnostics>   

How could I accomplish this?

You mean like when Exceptions happen or when the original app developer wrote to the Trace or TraceSource with a level of Error? They are different things and if the orig dev didn't catch all errors and write to trace with a level of Error, they you wouldn't expect to see errors this way. Also, if it is an unhandled exception, it certainly won't be traced. If you have the source code you might want to implement a global error handler like this: http://www.codeproject.com/Articles/14912/A-Simple-Class-to-Catch-Unhandled-Exceptions-in-Wi

Application.ThreadException and Application.SetUnhandledExceptionMode seem to be the relevant events.

If you don't have that, you might want to write a custom trace filter, which will allow you to send everything (Verbose) to the event log, but filter out all the messages without a substring of "Error" or what ever it is that distinguishes an error-type message from a non-error type message. (And again, this assumes that there is a catch block and in that block there is a Trace or TraceSource write statement with error info)