if karthus is killed first in bellows and then 2nd bellows retrigger targets someone else with deathknell does that karthus still proc it

Yes — the second unit's Deathknell still triggers an extra time.

Bellows Breath's Repeat executes the whole effect a second time during the same resolution (Rule 820.1.d), and Bellows Breath resolves fully before any cleanup. Karthus has lethal damage marked but hasn't been moved to the trash yet, so he's still on the board and his passive ("Your [Deathknell] effects trigger an additional time") is still applying when the second unit's Deathknell trigger is created. In fact it would work even if both deaths were truly simultaneous (FAQ #10742 / #5353) — passive abilities don't need to survive their own death.

The contrast case: if Karthus had been killed and trashed by a separate, earlier spell or ability (with a cleanup already resolved in between), his passive is gone and the later Deathknell would only trigger once. New choices for the repeated execution — including a different unit — are legal (Rule 820.2.a).