Quoting Michael Paoli (Michael.Paoli@cal.berkeley.edu):
One approach is have "talk" subscribed to "announce", and then remove from "announce" those that are subscribed to "talk" ... but having a list subscribed to a list tends to cause its own sets of issues/problems (we probably wouldn't want to do that without well testing it out and making sure we had any serious issues reasonably addressed).
FWIW, the reason I mentioned that tactic is that it's what SVLUG does, for the lists I admin for them. (I didn't create that setup, but inherited it.) I can't think offhand of any adverse side effects.