If I have 2 Glasc Mixologist die at the same time and Deathknell trigger go on chain, the first trigger bring back Karthus, does the second trigger will be affected by Karthus ability?

No. The second Deathknell trigger will still resolve only once, unaffected by Karthus.

When both Glasc Mixologists die simultaneously, the game checks the board state at that exact moment. Since Karthus, Eternal isn't on the battlefield yet, each Mixologist's Deathknell triggers only once (the base trigger). Those triggers become pending items and the number of resolutions is locked in (Rule 808.1.d.2, FAQ #9606).

When the first trigger resolves and brings Karthus back, he arrives too late — the second trigger was already created and its trigger count was fixed before Karthus ever entered play. Karthus must be on the board at the moment of death to modify those triggers (FAQ #10300).