After using Phantom Roll, the Double-Up icon appears on your screen (a die with a plus sign over it), signifying that the roll is eligible for Double-Up. Using Double-Up at this point will generate another random number from 1 to 6 and add it to the previous Phantom Roll total.
If the total comes to 12 or higher, the roll will Bust. Anyone in range of the busting Double-Up will have the roll effect removed from them.
You can continue to Double-Up on the same roll as long as the Double-Up icon is displayed on the screen, which is up to 45 seconds after Phantom Roll was initially used, or until you Bust.
A total of 11 creates the strongest effect. Hitting the lucky number for the roll is the next strongest effect, with lucky numbers falling between 2 and 5. Unlucky numbers give the weakest effect and are between 6 and 10. For most rolls, the unlucky number is equal to the lucky number + 4.
Double-Up can be used to apply a roll to a party member(s) that were not in range of the original Phantom Roll. Likewise, it is possible for someone that was in range of the original Phantom Roll to miss the Double-Up. If a Bust occurs when this happens, that person will still keep his roll effect.
The merited ability Snake Eye modifies Double-Up, forcing the next usage to produce a roll of 1.
If the eligible roll is dispelled during the double-up phase, using Double-Up will cause a roll of 0 and use the animation for Berserk.