No offence taken acelockco. But I can't help being "unrealistic" about my air management. My air consumption has been the same since Day 1. So I guess I should take more SCUBA classes since the main indicator of skillz improvement is air consumption and it's been unchanged. Therefore I've obviously not improved as a diver.

No one has taught me this specific style of sharing air. I have a lot of it, more than I could "realistically" use. So I really don't see any harm in sharing any of it, regardless of whether it's a drill or not.

My original question was regarding "why" I was advised not to use the same tank twice -- logic and a basic understanding of psi vs. tank capacity didn't make sense to what they were telling me.

As for hitting the NDL, I have done it every time since my first checkout dive & can't currently see a reason where I won't continue to do so. As for a penetration dive for the dive profile I had used as an example, I have hit my NDL twice on the same tank multiple times with approximately 900psi (+/- 100) when allowed to do so.