No one knows the correct percentage but at 100% Double Rate you can expect two characters (or more if Lucky) to hit for double or triple each turn in a personal basis of 85% or so, after many raids and being done with the event already I can tell you that I can count with my hands the times that I haven't hit double or higher with at least two characters, even though there's a slight modifier since I run Mordred
Triple rate up isn't reliable and only works when in combination with the other, even though this event due to popular request from the members of my Union I've activated healing/regen bonus instead of it, I just wouldn't recommend it
furthermore skills as Orleans Call, Brynhildr Attack buff and rate up for the party or Eidolon effects like Garuda/Anzu have a higher effect than those buffs but that's to be expected, while stacking two of this or more it just skyrockets and most of the party attack for double or triple (this only for the three turns they last, elementary)
I've also been testing the small theory that auto attacking increases the chances of a double attack for a few weeks now and even though vague for now it seems it works but it needs further testing