Does Hidden Stand United apply to all battlefields?

Yes, Stand United played from Hidden applies its +1 buff bonus to all battlefields.

Here's how it breaks down:

1. Targeting part ("Buff a friendly unit") — when played from Hidden, this target must be a friendly unit at the same battlefield where it was hidden (Rule 811.1.d.2).

2. Global effect ("Buffs give an additional +1 might to friendly units this turn") — this has no targeting restrictions and applies to all friendly units with buffs everywhere, on both battlefields (Rule 811.2 example, FAQ #851, FAQ #1457).

So you pick your buff target locally, but the +1 bonus boosts every buffed friendly unit across the board.