Several cards have globals or abilities that can only be used when something else happens first. These include the global abilites of Vibranium Shield, War Machine, Human Torch - "Flame On!" and Iron Man - Inventor as well as the text ability for Phoenix - Redd (and I am sure I missed a few more).
How many times can you pay for such abilities when there is a single trigger? And is that different for global abilities vs. text abilities?
Example of Play:
I successfully attack my opponent directly with a single Angel. If I have two unspent [shield] energy, can I use War Machine's global ability twice to gain a total of 2 life?