If there is multiple instances of the Deathknell of Ruined Rex, do I have to choose the target before resolution or when the Deathknell are put in the chain?

Based on the official rules and RiftJudge FAQ #10553, here is the breakdown of how your scenario works:

The Short Answer

You do not choose all targets upfront. Each instance of a Deathknell trigger is a separate item on the chain. You choose the target for a specific Deathknell trigger at the moment that specific trigger is finalized and placed onto the chain.

The Sequence of Events

When multiple Deathknell triggers occur (for example, if you have multiple copies of Karthus, Eternal on the board), the sequence follows these steps:

1. Deathknell Creation: The triggers are generated by the death of the unit (Ruined Rex). Per Rule 808.1.d.2, these become Pending Items.
2. Finalization: You move to the Finalize step. You place the Deathknell triggers onto the chain one by one.
   • As you place the first trigger onto the chain, you choose its target.
   • As you place the second trigger onto the chain, you choose its target, and so on.
3. Chain Resolution: The chain resolves in LIFO (Last In, First Out) order. You will resolve the first trigger completely (dealing its damage) before addressing the next one.
   • Crucially: After each individual trigger resolves, the state becomes OPEN, giving your opponent an opportunity to play [REACTION] cards before the next Deathknell trigger begins to resolve.

Summary of Rules

• Targeting: Per Rule 355.14.b, targets are chosen when the ability is finalized on the chain. Because each trigger is a distinct item on the chain, you make these choices sequentially as you place them.
• Priority: Your opponent has priority to add reactions to the chain after every single item resolves. You do not get to resolve all your Deathknell damage at once before your opponent can interact.

This is confirmed by RiftJudge FAQ #10553.