When damage is dealt, the attacking player/s choose a specific opponent that each of the unblocked creatures damage. In this case, Hypnotic Specter will only damage one opponent, and only that player will discard a card, not both.
806.7f - As the combat damage step begins, the active team announces how each attacking creature will assign its combat damage. If an attacking creature would assign combat damage to the defending team, the active team chooses only one of the defending players for that creature to assign its combat damage to. Then the defending team announces how each blocking creature will assign its combat damage. See Rule 510.1.
