Come on, you know it would work. The departing aircraft taking off on 21 rolls past the intersection and the one ALREADY lined up and WAITING on 24 can blast off. Diverging tracks. The 21 departure ideally is a left turn and 24 is the right turn. Arranged that way on clearance receipt.
Good luck with that, we'll get you to negotiate the environmental issues (noise) with all and sundry. Wouldn't take long to have every MP, Councilor etc up for re-election to make it their mission to have restrictions and a curfew for PH.
Getting back to the actual suggestion, how does the 24 departure get to the threshold? Cross 21, another chance for pilots to take their sweet time, doodling along. It would just create more apron conflictions around D or if they come out B or W you would lose slots while they cross, you really have no idea.