If Karthus and Watchful Sentry die at the same time to Falling Star, do you still get the effect from Karthus?

Yes, you still get the effect from Karthus, Eternal.

According to RiftJudge FAQ #9028 (and similar FAQs #9890, #8974, #9711, and #9317), 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 other unit's (in this case, Watchful Sentry's) Deathknell trigger is created, even though Karthus himself is being killed simultaneously by the Falling Star.

The Interaction

1. Resolution: Falling Star resolves, dealing lethal damage to both Karthus and Watchful Sentry simultaneously. Both units die.
2. Passive Persistence: Because Karthus's ability is a passive, it remains active as the Deathknell trigger for Watchful Sentry is placed on the chain.
3. Result: The Watchful Sentry’s Deathknell effect triggers twice (once for its own death, plus one additional time due to Karthus’s passive effect). You will draw 2 cards total.