In addition to the runway crossings mentioned by Zooker in Post 7 Manchester's departure routes, for environmental reasons, are not particularly good for capacity with many consecutive departures requiring longer separation intervals. Also the taxiway and stand infrastrucure could not cope with capacity levels achieved at Heathrow without getting totally clogged up. So to achieve the best capacity out of two runways you need the whole airport infrastructure and ATC procedures to be designed to match it.