Hi gearpins,
however if I were to loose left bleed in flight my check is happy to allow the right bleed to supply the right pack!!
The QRH is written as a simple practical solution for a single sector when work load might be high.
The MEL OPS is written considering the chance of an another independent failure to the air supply system occurring during a permitted series of flights. There is no time pressure when the aircraft is on the ground (except commercial) so a more complicated working solution is often offered for consideration.