I don't think they need to decide anything just yet; for once I agree with David Coulthard. They just have to see how the next two races play out, and even then they may not be in a position to do anything.
As Martin Brundle pointed out, Alonso's so-called 'advantage' in being the clear No. 1 driver at Ferrari, is a bit meaningless in practice because Massa is too far behind anyway to be of much help. Today was Felipe's best result for a while yet he still got nowhere near Alonso, and took no points off the opposition (he was always behind both Red Bulls and Button ruled himself out of it).
Just what are Mclaren or Red Bull supposed to do? for instance, Hamilton has consistently outqualified and outraced Button, so in what way is Jenson supposed to 'support' Lewis (other than try and take points off the opposition, which I presume he is trying to do anyway)?
The only practical way these teams can prioritise Webber or Hamilton is if Vettel or Button find themselves directly ahead of their respective teammates and therefore in a position to make a switch. At Mclaren at least, that doesn't look very likely at the moment.