[zombie-brand-facepalm]
Sorry, I think i wasn't clear. Let me rephrase.
''You technically could get 2 banshees within a single minion wave''
I did not mean that in the literal way, I meant that with a 20 second cooldown, you could technically get the shield twice within a single wave. Since minion waves spawn every 30 seconds. Which meant that with one minion wave sieging, the defending team could avoid alot of diving.
Sorry I wasn't too clear haha
EDIT: And iirc, I think that is exactly why Riot made the cooldown longer. I think it was in the tl;dr in patch notes.