Can a hidden card played from battlefield A target enemy units at battlefield B when battlefield B goes into showdown?

Ruling: A spell played from hidden has the additional targeting requirement "here," so you cannot target cards at battlefield B with a card hidden on battlefield A (unless it's a card with special targeting restrictions like Tideturner).

Nuances:

• If a unit on battlefield A with a hidden card is targeted by a spell from battlefield B, you can react with your hidden card on battlefield A to pump that unit before the spell resolves by adding your hidden card to the chain.