From all the posts here, it doesn't seem this was discussed..
What if I open a chest, obtain the RARE/EX, can I be the one to open another chest in a party and would the RARE/EX still pop?
From what I can tell by reading this thread, two people in a party, 1 person opens chest, gets RARE/EX, they continue partying, then the 2nd player opens chest and gets the RARE/EX.
What if Player 1 opened both chests? Would it still work?