Getting the Ant <modified> selector to work properly

Question

I have a target in ANT that needs to run a compiler on a given set of files twice: once for debug, once for production. I only want to run the compiler if the source file has changed, so I set up a < modified> selector. However, since I need both the debug and prod task to run for a given modified file, I set the 'update' property to false on the first run. I have something along the lines of:

    <!-- Do the debug build -->
    <apply executable="compiler">
        <fileset dir="${js.src.dir}" includes="*.js">
            <!-- don't update the cache so the prod build below works -->
            <modified update="false" 
                seldirs="true"
                cache="propertyfile"
                algorithm="digest"
                comparator="equal">
              <param name="cache.cachefile" value="cache.properties"/>
              <param name="algorithm.algorithm" value="md5"/>
            </modified>
        </fileset>
        <args for debug build/>
    </apply>
    <!-- Do the production build -->
    <apply executable="compiler">
        <fileset dir="${js.src.dir}" includes="*.js">
            <modified update="true" 
                seldirs="true"
                cache="propertyfile"
                algorithm="digest"
                comparator="equal">
              <param name="cache.cachefile" value="cache.properties"/>
              <param name="algorithm.algorithm" value="md5"/>
            </modified>
        </fileset>
        <args for prod build/>
    </apply>

However this is not working. My first call to the compiler ends up updating the cache anyway, and the second call gets skipped. What am I missing here?

UPDATE: I got around the problem by using the < depend> selector instead, but still curious how to accomplish the same using < modified>