re shifting both simultaneously, I actually dont do both exactly at the same time, but with a slight slight delay. I used to shift dt shifters teh same, with my right hand doing both levers. Part of my hand would do the front downshift and then the other half of my hand would do the one or two upshifts at the rear.
I'd describe the two shifts as overlapping each other in a slight way.
with STI or trigger shifters, I do the same, shift the chainrings and then a fraction of a second later do the rears, to make sure that the chain tension is constant for the front shifting, which I find to be better overall.
but its all about timing with this sort of stuff isnt it?. Kinda like back in the day shifting a motorcycle on track without using the clutch, it was all about exact timing of backing off the full throttle for that fraction of a second, and timing both the upshift and back on the throttle again in a smooth yet super fast way.
and you aint gonna learn all this stuff or get good at it reading about it on the interwebs, you have to have a feel for it.