Can you play the effect of a hidden card targeting the other battlefield?

Ruling: No, hidden cards cannot target units or make choices at a different battlefield than where they were hidden. All choices must be made at the battlefield where the card was hidden, if able.

Nuances:

• Some cards like Tideturner can get around this restriction because they specify "another" battlefield in their text
• Some cards like Stand United still have effects beyond their targets that can apply elsewhere
• Hidden cards can still be played from one battlefield to react to events at another battlefield, as long as the card doesn't require targeting (you can play a hidden card from battlefield 1 during a showdown at battlefield 2 if it doesn't need to target)
• Hidden cards cannot be played the same turn they are hidden
• The restriction on hidden cards only applies to their targeting, not to when they can be played as reactions