Let's pretend that I have 2 {Possibility Storm}'s out. If you cast an instant spells, will this trigger twice letting you cast 2 cards out of your library? Triggering say {Guttersnipe} 3 times?
You got it! Possibility storm doesn't care if you actually exile the original cast spell to go rifling through your library, only that you cast it.
That's what I thought. My gears have been turning to make a deck that abuses the chaos that the storm creates. I'll post the results.
Relevant ruling, if anyone cares:
4/15/2013 If Possibility Storm's ability doesn't exile the original spell (perhaps because another Possibility Storm already exiled it), you'll still exile cards from the top of your library until you exile a card that shares a card type with it and have the opportunity to cast that spell.