If you control two Zhonyas Hourglass and two units die simultaneously, can both Zhonyas save one unit each?

Ruling: Yes, if two units die simultaneously and you control two Zhonyas Hourglass, each Zhonyas can save one unit. X Zhonyas can save X units, even if they die simultaneously.

Sequence:

• When two units die simultaneously, each Zhonyas sees both death events
• The controller picks which death event each Zhonyas will replace
• Each Zhonyas replaces one of the deaths and is killed instead
• Both units are saved

Nuances:

• If only one unit dies with two Zhonyas in play, the controller chooses which Zhonyas replaces the death. The second Zhonyas does nothing because the death has already been replaced by the first Zhonyas.
• When multiple replacement effects would replace the same event, the owner of the object being acted on determines the order the replacement effects apply.
• The second replacement effect cannot replace a death that has already been replaced.