if i use bellows breath on 2 units at a battlefield. and they use emperor's divide. now they're at different locations. how is it decided which still get hit?

You decide. The "same location" requirement is a targeting restriction when Bellows Breath is cast, not a resolution requirement — so the spell doesn't fizzle when Emperor's Divide scatters them.

On resolution, you (the spell's controller) choose one location where your originally targeted units currently are. All originally targeted units at that chosen location take 1 damage; any at other locations are unaffected. You can't split damage across locations, and you can't pick new targets — only units you originally chose.

One practical note: in 1v1, Emperor's Divide sends both units to their controller's base, so they'll both usually end up at the same location and both get hit. "Different locations" only matters if the two units have different controllers (e.g., 2v2, where each moves to its own owner's base) — then you pick one to hit. (Rule 355.11.b; FAQ #7422)