How often does anyone actually shift front and rear simultaneously? I am curious as I never do that currently. As for sequential, even with electronic shifting it is slower to shift the front then the rear, and not as smooth. I would still prefer to know when an FD shift was coming.
Happens quite often if the grade slowly changes from flat to uphill. For example, suppose you have 50-34 front / 11-28 rear (11,12,13,14,16,18,20,22,25,28). You're on a flat in 50/18 gear (19.5 mph @ 90 rpm). As the grade increases, you downshift to 50/20 (17.6 mph), then to 50/22 (16.0 mph). What do you do next? Any further in the back and you'll be cross-chaining. If you shift in front, 34/22 is 10.9 mph and that may be slower than you want. So you downshift in front and at the same time upshift in the rear twice, to 34/18 (13.3 mph @ 90 rpm).
More generally, the problem is that 50-34 gap in front is equivalent to 3 to 4 gear gaps in the rear, especially with a tight cassette and lots of cogs (up to 5 gaps if you have a 12-25 11-speed cassette) so you often have to compensate for the front shift with one or more shifts in the opposite direction in the rear. The problem is lessened with 53-39 in front but it's still there.
That is a good description nameless, though in reality it's easy to have a slightly too high or too low cadence for a second or two and shift the front and rear sequentially rather than simultaneously.
I think that's what most good riders do to minimise chain drop risk.
So in your situation I would shift up at the back (22 to the 20), then shift down at the front (34/20), and then shift up again at the back to whatever sprocket gave the right cadence