If both players are running Grimme as their Legend, does Grimme's trait apply once or twice?
Grimme:"When attaching a spell to a card, increase the spell's ghost rock cost by the number of copies of that spell in play."
Many thanks for the question.
Legends only affect the owner unless they specifically say otherwise. As such the trait only applies once.
You do however have to check the total number of copies in play attached to all cards.