We have a custom 404 page handler that routes to different domains based on the request

We used the post below

http://our.umbraco.org/wiki/how-tos/how-to-implement-your-own-404-handler

However this setting takes precedence

<errors>
            <!-- the id of the page that should be shown if the page is not found -->
            <!--        <errorPage culture="default">1</errorPage>-->
            <!--        <errorPage culture="en-US">200</errorPage>-->
          <error404>3343</error404>
        </errors>

Is there any way to make a custom 404 for 2 sites on the one instance of Umbraco???