if i have astrol heron. and play a spell. then they react to it, and i react to their reaction, i now have two on chain. but i paid full price for both. what happens? What gets discounted?

Neither of them gets discounted — paying full for both was correct.

Heron's trigger ("When you play your first card each turn") only fires once your first card finishes resolving (FAQ #11610), so that spell is the trigger, not the discounted card. Your reaction spell was played before the trigger resolved, so the discount effect didn't exist yet — full price for both was right.

When the trigger finally resolves, it creates "[2][A][A] off your next card," and that applies to the very next card you play after the chain clears (Rule 356.4). It's mandatory and hits exactly one card — you can't save it or split it across two (FAQ #11670).

One caveat: if the Heron trigger had already resolved earlier in the turn, then the first spell you played should have been discounted automatically — never the second one on the chain.