If I'm holding 2 locations and I put Tide Turner hidden on location B and then it's the enemy turn and they contest Battlefield A, can I Tide Turner reveal location B and swap my Battlefield A and B unite?

Yes — you can. Playing a hidden Tideturner from Battlefield B to swap with your unit at Battlefield A during the opponent's Showdown at A is fully legal.

Here's how it works: Tideturner played from hidden has Reaction, so you get a window to play it during the Showdown at A — either by responding to something on the Chain (the attack trigger, an action-speed effect) or when the opponent passes priority to you. Its errata'd "When you play me" trigger lets it target a unit you control at any location, bypassing the normal hidden targeting restriction (FAQ 11326).

When it resolves, Tideturner moves to A and your unit moves back to B. Since B is a battlefield you control, no new Contested is applied there, and A is already Contested by the opponent — the Combat at A simply continues with Tideturner as your unit. The swap happens as one uninterrupted resolution, so you can't lose control of A mid-swap.