I have no idea, since nobody's done this one, and BELIEVE ME I've thought about it as a suggestion, but I Couldn't find such a thing either..
The easiest way would be a DVI Y-adaptor.. but no such thing exists *nod* you might have to hack a cable to do that.. your tricky part will be figuring out which pins carry the extra signal, they ARE the middle 6 I can tell you that much, but you might have to rip apart a real "DVI to composite" (or DVI to svideo) adaptor to figuree out which goes where
