Quote from WizardMN »Necromancy will trigger twice since it is a permanent entering the battlefield. Each trigger will resolve separately. The first trigger resolves and brings back the target and Necromancy attaches to it.
Then, the second trigger returns the second target and attaches to the new creature. Since Necromancy says "When Necromancy leaves the battlefield, that creature's controller sacrifices it" and it hasn't left the field yet, the first won't be sacrificed.
You will end up with both creatures until Necromancy leaves the field.
Of course, there is always the caveat that new rules will change how this works.