Can anyone suggest ANY metropolitan area which has made a success of connections between different airports ?
There I was thinking the £6 Billion ThamesLink project will connect Luton & LGW, whilst the £15 Billion Crossrail project will connect LCY and LHR !! Perhaps you should visit Farringdon which is being built as an underground interchange station between these two lines.
If Boris gets his way on Crossrail 2 then the Crossrail 1 re-build of Tottenham Court road is already making passive provision for it, as a Crossrail 1/2 interchange. It's also worth remembering that Crossrail 1 will run through Liverpool Street station, home of the Stansted Express.
Of course that doesn't extend to LGW actually contributing to the cost (à la BAA/Airtrack).
Well, they are contributting to the present Gatwick Airport station upgrade, and I have no doubt Network Rail would ask them for a LGW - Guildford contribution, if BAA contributed towards the Guildford - LHR section.