If multiple Glasc Mixologist die at the same time, can you bring Karthus back from one of them and then Karthus adds triggers to the other deathrattles on the chain?

No, you cannot bring Karthus, Eternal back from the trash using one Glasc Mixologist's Deathknell and then have him increase the number of triggers for the other Deathknell effects already on the chain.

Why this doesn't work:

1. Timing of Triggers: According to Rule 808.1.d.2, Deathknell triggers are added to the chain as pending items before the units are moved to the trash.
2. Snapshot of State: When the Deathknell abilities trigger, the game notes the board state at that moment to determine how many times they trigger. Because Karthus, Eternal is on the board at the moment of death, he modifies the number of triggers for all Deathknell effects that trigger at that same time.
3. Chain Resolution: Once the Deathknell triggers are added to the chain, the number of times they will resolve is already locked in based on the board state at the time of death. Playing a new Karthus, Eternal from your trash after the triggers have already been added to the chain does not retroactively increase the number of times those existing triggers will resolve.

Sequence of Events:

1. Simultaneous Death: Multiple Glasc Mixologist units die simultaneously while Karthus, Eternal is on the board.
2. Trigger Addition: Because Karthus, Eternal is on the board, each Glasc Mixologist's Deathknell triggers twice (the base trigger + one additional time from Karthus's passive).
3. Chain State: These triggers are added to the chain as pending items. The total number of resolutions is now fixed.
4. Resolution: You resolve the Deathknell triggers one by one. If you use one to play Karthus, Eternal from your trash, he enters the battlefield, but he is too late to affect the Deathknell triggers that were already placed on the chain.

This is my interpretation based on the rules regarding Deathknell timing (Rule 808.1.d.2) and how passive abilities interact with triggered abilities. No official FAQ exists for this specific interaction, but it follows the established logic for how Deathknell triggers are locked in at the moment of death.