say someone spends a dark fate to make a character who has the Orb of Destiny reroll their die.

does that character roll 2 dice, as the Orb of Destiny indicate, or does it only work for white fate that the character with the orb uses?

same question for other object & spells, such as Twist of Fate. if someone uses a dark fate on a character, can that character cast Twist of Fate to choose his result?

Cast after you pay a fate but before you reroll the die.
You choose which die result you will use instead of rerolling it.

When you pay a fate to reroll a die, you may roll 2 dice and choose which result to use. If you roll doubles, discard all of your fate immediately along with this card.

You didn't pay a fate in either case so I'd say no.

Yeah, but those cards were printed before the concept of dark fate existed, so I'm not sure that the "You" part should be taken into consideration. Principally, they're supposed to work when a fate is spent and the player has to reroll his die, so it shouldn't matter who spent the fate, as long as the player who has the item / spell has to reroll, no?

Well.. interpreting rules as how we think they were supposed to work never end in something definite. You could just as well put it the other way around, that this is the way it's supposed to work.

Therefore, when in doubt. Just do exactly as the cards say. I dare say that this is the way almost every question ends in this forum (when the possibility exists).

Yep roll with what the card says until otherwise printed in a faq.

You means you. End of. No using the Magic Object or Spell...

I guess ffg has been working with the idea of how to use light and dark fate since they introduced the two-sided tokens. Maybe that's why the mentioned cards focus on "you" rather than "fate" or "roll".