The bleed sides can then be split allowing pack on in hot weather whilst starting the other one.
That's what we do in the 717. Start left/1, ISOL>Auto ie closes, then left pack on, then start right/2. Aircon whilst we wait eons for second engine to start is good!
you could be winding up 3 and 4 whilst the last pax were boarding, main pax door being closed and the steps were being taken away.
I imagine those silly Health & Safety Rules prevent us from doing similar these days.
We don't do that as if an evacuation was required eg engine blowup/fire on start none of the slides are armed.
