can you react to astral heron trigger, can you play a reaction speed card between 2 heron triggers on Bfs assuming they both flat the trigger already

It depends which "between" you mean — you cannot insert a Reaction between the two Herons triggering, but you can play one between their resolutions.

Both Herons trigger simultaneously when you play your first card, so they go on the chain together as pending items, with you choosing their order (Rule 383.3.d). Finalizing doesn't pass priority (Rule 337.1.a), so there's no window to act until all pending items are done — the opponent can't slip a Reaction in ahead of the second trigger being queued.

Once the first Heron trigger resolves, though, priority goes to the controller of the next item on the chain (Rule 340.4), so you get a genuine window before the second trigger resolves. The chain is non-empty there, so it's a Closed State: only Reaction cards/abilities are legal.

One real consequence: a card played in that window is "the next card you play this turn," so it would consume the first Heron's discount, leaving the second Heron's discount for the card after that — you'd have to actually want that. If your goal is stacking both discounts on one card, just pass and let both triggers resolve (FAQ #11679).