A player can use a Cursed card to copy the trash effect of another card that is being trashed in the same phase, but the card being copied must still be in play (i.e., in the played card area) at the time the Cursed card is played.

This is supported by the following:

• On page 29, the FAQ states:

  "Can I use a ‘Cursed’ card to copy the Trash effect of a card?
  Yes, but remember that you would trash the ‘Cursed’ card as well. The best example of this would be trashing ‘Sprint’ for 3 and then copying it with ‘Cursed’ for another 3 . This would give you a total of 6 and 1 . You would trash both the ‘Sprint’ and ‘Cursed’ card from your deck."

• On page 30, it is explained:

  "Every character has ‘Cursed’ cards in their starting deck that allow you to copy the effect of any other card you played this phase. This allows you to copy the more powerful cards that you are acquiring but will require you to increase your [Curse]. When you copy another card, remember that you must copy everything on it."

From these references:

• The Cursed card copies the effect of another card "you played this phase," which means the card must have been played and still be available to copy during the phase.
• The FAQ example specifically describes trashing Sprint and then copying it with Cursed in the same phase, resulting in both cards being trashed and both trash effects being resolved.

Summary

A player can use a Cursed card to copy the trash effect of another card as long as the card being copied was played this phase and is still in the played card area at the time the Cursed card is played. Both cards are then trashed, and both trash effects are resolved.

Source: page 29 (FAQ), page 30 (Cursed card rules)

Source: Core Rulebook (pp. 28, 29, 30, 4, 8)