A couple of points :
1. While I agree that a move of one of the alliances to LGW is most unlikely, I think it is wrong to assume that just because the airline market is configured in a particular way in 2015 it will look the same in 2025 and 2035. In a growth environment there is every reason for the market to change and for fourth and fifth forces to develop further than they have done so far.The case for LGW does not depend on some improbable event like Skyteam upping sticks occurring.
2. Let's suppose that the projects have to be commercial, at a minimum inside the perimeter fence and arguably including the effects on the M25, the Brighton Line etc. If that is the case, it's pretty clear that doing both projects simultaneously will lead to neither being commercially financeable. Therefore the most you could expect will be the go-ahead for one for completion in ten years time with an expectation of the second in twenty years time if UK and world income growth, carbon prices etc pan out as expected.