If I have a drawn out meal with multiple courses I simply inject for every new course. So occasionally it has happened I injected at 5 for a beer, at 5:10 for a snack to accompany the beer, at 5:25 for some more snacks because they were simply too good, at 6:15 for the soup, at 7 for the small pie, at 7:30 and 7:45 for the main course, again because the carby dish simply tasted too good, and at 8:30 for the ice-cream. As by that time, I didn't have a clue how far off I was with guessing carbs, the last 2 doses were smaller than what I thought I needed, so I corrected twice about 1 hour and 3 hours after.
Worked just fine, actually, but I'm happy christmas is only once a year
In normal every day life, I don't mind 'stacking' as long as I'm pretty much on top of my doses per food. I need the same amount of insulin for the planned and injected for cookie right after a meal, as for the same cookie an hour later. So it is the same if I take 8 units for meal + cookie or 6 units for meal and 2 units for cookie after an hour.
The main trick seems to be to keep in mind how much you have eaten and how much insulin you have on board, and how sure you are of your doses. I wouldn't pull tricks like that first meal if I hadn't planned to stay awake for at least 4 hours after to see what happened.