Can a player activate a hidden Tideturner from Battlefield A to swap units when combat is happening on Battlefield B?

Ruling: Yes, you can activate hidden cards regardless of where the showdown is happening. Tideturner specifically can target units on another battlefield because the card explicitly says it can.

Nuances:

• Hidden cards can normally only target units at their own battlefield unless they explicitly state otherwise
• Tideturner is one of the rare cases where a hidden card can target outside its battlefield because the card text allows it