iframe aus anderem iframe heraus ansprechen?

[ck]

In gekürzter Fassung ...

<!-- SUB-NAV -->
                <td><iframe name="subnav" src="subnav/subnav_off.html" width="430" height="15" frameborder="0" style="border-width:0px;" border="0"  scrolling="no"></iframe></td>

<!-- MAIN -->
             <td width="632" height="376" colspan="16">
<iframe name="main" src="main_off.html" width="632" height="376" frameborder="0" style="border-width:0px;" border="0" resize="no" scrolling="no"></iframe></td>
...

von SUB-NAV aus soll nun MAIN angesprochen werden.
weißt du weiter?