seriously speaking, when have anyone seen inter airport connection works. LHR/LGW, HND/NRT, KIX/ITM(These last 2 failed so badly that came at the growth of ICN, and now HND resumes its intl flights), AMS/CDG, and did MEL/SYD ever talked about cooperation? This is just utter nonsense for people who thinks airport can cooperate. Logistically speaking, shorts trips (besides and the 2 ends) are bad and why do you want to shove in a short trip when you can get a big trunk route elsewhere?
anyway, comparing a 3rd runway with the royally useless HSR, this is money well spend considering the coverage of a runway. The only real obstacle in between is the environmentalist vs runway discussion.