If I have resonating strike hidden in a battlefield that I control with 2 units, then another combat showdown starts at the other battlefield I also control. Can I reveal hidden strike and bring a unit from there to the other battlefield where the combat showdown is happening?

No. When you play Resonating Strike from hidden, the destination battlefield must be the battlefield where the card was hidden — you can't use it to move a unit to the other battlefield where the showdown is happening.

The card's "choose a battlefield you control" choice can be satisfied at the battlefield it was hidden on (it's a battlefield you control), so it must be chosen there (Rule 811.1.d.2). The "unit at a different location" choice can't be fulfilled at that battlefield, so that choice is free — but the destination is locked. In practice, revealing it moves a unit from another battlefield (or your base) to the hidden battlefield, not the reverse.

You can still reveal and play it during the showdown at the other battlefield — hidden cards are playable whenever you have priority — it just can't target across battlefields (FAQ #5820, Rule 811.1.d.2).