Just got Corsair to level 10 so haven't tried it in party situation yet. HEre's what I gathered on the roll, there are 2 options I figured could be valid:
1) There is a lucky and unlucky number, like stated before, where the first one gives you better bonus then the second one.
2) Blackjack type. The more rolls you get without busting, the better the bonus will be.
I tried to check exp bonus on EM mob that originally give 100 exp. With no double-up I was getting 105, 1 DU would give me 110 and 2 would give me 115. This leads me to believe each roll adds 5% to the stat/exp it affects. The more rolls you get in without busting, the better the bonus.
I may be wrong, still a bit early to be sure of anything, but that kinda makes sense.
HAs anyone figured out what the elemental cards are for yet?