Some cards, like Chittering Doom, care about the results of individual die rolls and not the total if two dice are rolled together. For example, if you use The Big Idea’s second ability to replace one die roll with two and roll two 3s, Chittering Doom’s ability (which triggers whenever you roll 4 or higher on a die) won’t trigger.

If another spell or ability instructs you to roll two dice and use the difference, you can use The Big Idea’s second ability to replace one of those die rolls. First, roll two dice and use their sum. That’s the first result. Then roll one die. That’s the second result. Then figure out the difference between those two results.

If you use the second ability to roll two dice instead of one, any ability that triggers “whenever you roll a die” will trigger twice.