, banish this card from your cemetery: Select an Amaterasu and Tsukuyomi in your cemetery and summon them.
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Search your deck for an Amaterasu or Tsukuyomi, summon it, then shuffle your deck.
QI have 4 cards on my field. Can I select Amaterasu but not Tsukuyomi, or vice versa, and play this spell's activated ability?
ANo, you must select both. Since you have 4 cards on your field, after selecting both, you choose one of them to summon.
QIf I have Amaterasu or Tsukuyomi in my cemetery but not both, can I still play this spell's activated ability?
ANo, you must be able to select both.
QCan I only play this spell's activated ability when it's in my cemetery?
AYes, that's correct.