If I have two Astral Heron triggers on the chain, can I play a reaction speed spell to get the discount off the first Heron then get another card discounted before it resolves?

Yes — you can split them across two cards, because each Heron's discount only exists once that trigger resolves.

After the first (top) trigger resolves, there's a priority window before the next item resolves (Rule 340.4 / FAQ #1179). Playing a Reaction card there makes it "the next card you play this turn," so it gets that Heron's [2][A][A] off. When the second trigger then resolves, it creates a fresh discount for the next card you play. A Reaction is required in that window, though — the chain isn't empty, so only legally-timed (Reaction) cards can be played.

The thing you can't do is discount a card played while the triggers are still on the chain — the discount doesn't exist yet (FAQ #11689). And if you let both triggers resolve before playing anything, the two discounts stack onto one card (4 off total) rather than covering two cards (FAQ #11679).