I've never read of the both scenario ever being proposed, but can't understand why?
Growth forecasts at LGW upon which the business case for a second runway depend upon, are based on there not being an overnight huge capacity increase at LHR and consequently losing a substantial part of their existing traffic to LHR.
There's two sets of LHR carriers with existing and concurrent operations at LGW, those who are there simply because they can't grow at LHR and those who offer service in a complimentary fashion.
Virgin and BA long haul fall into pot one, BA short haul probably into pot two. (debateable). Aer Lingus serve the region in addition to LHR, whereas Air China, Icelandair and TAP might be expected to consolidate to LHR. Vietnam would also be a better fit for LHR Ithink. There is then the danger that LGW would remain doing what it has always done so well, taking holidaymakers to the sun, with a core easyJet, Monarch, Thomson and Thomas Cook operation. I suspect Norwegian might be tempted by LHR. In any case, constrained capacity at LHR drives LGW growth, if that restriction goes, much of the strength of LGW's growth, perhaps all of it in the short term, is lost.