If Karthus and Watchful Sentry die at the same time, do you draw 2 cards?

Yes, you draw 2 cards.

According to RiftJudge FAQ #10438, Karthus, Eternal's ability ("Your [Deathknell] effects trigger an additional time") is a passive ability, not a triggered ability. Because it is a passive, it remains active at the exact moment the Deathknell trigger for the Watchful Sentry is created, even though both units are being destroyed simultaneously.

Sequence of Events

1. Resolution: An effect (such as Falling Star) deals lethal damage to both Karthus, Eternal and Watchful Sentry simultaneously.
2. Passive Persistence: Because Karthus's ability is a passive, it remains active at the instant the Watchful Sentry's Deathknell is added to the chain.
3. Result: The Watchful Sentry's Deathknell effect ("Draw 1") triggers twice: once for its own death, and one additional time due to Karthus, Eternal's passive. You draw 2 cards total.