Here is the idea behind insulin stacking…
You know how your insulin lasts several hours? Maybe it takes 4 hours before your bolus is completely gone. Most of it gets used in the first 60-90 minutes, but some of it is still trickling through your body for a total of 4 or 5 hours.
Suppose you take insulin for your meal, and then an hour later you want to eat more, so you take more insulin.
The endo is worried that you will still have insulin from your first bolus, and the second bolus will add to the first bolus with the total amount of insulin, the IOB you still have. That is called “stacking”.
So if you were a new diabetic, and you didn’t know what you were doing, he would tell you to not stack, because the second dose would get added to some of the first. The second bolus would add to your IOB, and you might go low.
But…you know what you are doing! You test a lot. You pay attention to your numbers. You are not a new diabetic!
If you calculate the first bolus correctly, and you want to add more food later, of course you should take more insulin for that.
Keep in mind, your IC ratios might be different if you have a bit of insulin left over from the first bolus. You may need a bit less insulin for the food you eat later, just because of the leftover insulin.
For someone like you, with your experience, this is just silliness. If you want to eat a little more later, of course you can take insulin for that. Just pay attention to possibly different IC ratios, test, and pay attention to your BG.