If I put Sacred Shears on a unit and I have Kartus and I kill my unit, will Sacred Shears create two Deathknell triggers drawing me 2 cards?

Yes, you will draw 2 cards total.

According to RiftJudge FAQ #765, when a unit equipped with Sacred Shears dies while you control Karthus, Eternal, Karthus's passive ability causes the Deathknell effect granted by the Shears to trigger twice.

The Sequence of Events:

1. Death: The unit equipped with Sacred Shears dies. The unit’s Deathknell (provided by the Shears) is added to the chain as a pending item while the unit is still on the board.
2. Karthus's Passive: Because Karthus's ability ("Your [Deathknell] effects trigger an additional time") is a passive ability, it is active at the moment the Deathknell trigger is created.
3. Resolution: The game processes the Deathknell trigger twice due to Karthus's passive, resulting in you drawing 1 card for each resolution, for a total of 2 cards.

As clarified in the FAQ, this interaction is specific to Karthus's design as a champion who modifies how these triggers function at the moment of death.