I don't know how serious the proposal to use the "emergency runway" in addition to the main is, but I cannot see any possibility of it ever being used simultaneously. It surely is too close to the main & it is too close to the taxiway. Even if they were both to be used simultaneously, how would a/c get to & from the main runway without crossing the "emergency runway" ? Surely this would remove any extra capacity expected to be generated by the use of the "emergency runway". Similar problems would also be caused by the proximity of the taxiway.
The CAA have a good record of abandoning long standing safety regulations to accommodate the commercial needs of operators & ATC, but I cannot see it countenancing this half baked suggestion.