Tight Slot & LHR Director - I think what Nigel O.D. is saying is that once we've made the decision to divert in a low fuel situation we have commited ourselves to the runway to which we are diverting - we do not carry fuel to divert from the diversion.
Under certain well-defined conditions it is quite permissable, and dare I say prudent, to use your destination's 2nd runway as your diversion, rather than to set sail for a single runway airport.