When two units would die simultaneously to a spell like Falling Star (which deals 3 damage to two units), and I react with Zhonya's Hourglass, which unit gets saved?

Ruling: Falling Star creates two separate triggers on the stack, not simultaneous deaths. You can choose which unit to save depending on when you activate Zhonya's Hourglass.

Sequence:

• First instance of 3 damage resolves, killing one unit
• You can activate Zhonya's Hourglass to save that unit
• Second instance of 3 damage resolves
• Alternatively, let the first unit die, then activate Zhonya's to save the second unit

Nuances:

• If Zhonya's is face-up at the battlefield, you cannot choose not to use it - it automatically replaces the death of the first unit
• If Zhonya's is hidden, you can choose to flip it after the first unit dies
• If the first unit to die is the only unit at Zhonya's battlefield, you must use Zhonya's before it dies or you'll lose control of the battlefield and Zhonya's goes to trash