In every zone other than the stack, Twice Upon a Time uses only its non-Adventure characteristics, and you ignore its alternative characteristics in those cases. For example, while it's in your graveyard, its mana value is 6.
Twice Upon a Time // Unlikely Meeting
If an effect copies an Adventure spell, that copy is exiled as it resolves. It ceases to exist as a state-based action; it's not possible to cast the copy from exile.
You must still follow any relevant timing rules for the non-Adventure spell you cast from exile. Normally, you'll be able to cast it only during your main phase while the stack is empty.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Elizabeth Peiró
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