which deatknell goes first rift heralds dies on same time in combat

It depends on who controls them — the player(s) controlling the Heralds choose the order, and they resolve in reverse order (last placed resolves first).

Both Deathknells trigger simultaneously when the Heralds die together (they become Pending Items and sit until Combat Cleanup finishes), so the ordering rules apply: if one player controls both Heralds, that player chooses which goes on the Chain first — meaning the one placed last resolves first (Rule 383.3.d). If you and your opponent each control one, the Turn Player orders their trigger first, so the non-turn player's Deathknell goes on the Chain last and resolves first (FAQ #2828, Rule 383.3.d.1).

Note the practical difference is small: each Deathknell plays a unit to its controller's base, so ordering only matters for how the units enter relative to each other and any Play Effects they have.