If I have two buffed Setts on board and my opponent plays Icathian Rain dealing 6 damage to both, can I save both Setts using their abilities?

Ruling: No, you can only save one Sett. If multiple units would die at the same time, you as their controller can pick one of them to save, and the rest die.

Sequence:

• The Kai'Sa player assigns the order of each individual reflexive trigger from Icathian Rain before they're added to the chain all at once
• The triggers resolve one by one in reverse order
• One Sett will die before the other (Icathian Rain does not deal damage simultaneously)
• You can only save one of the Setts

Nuances:

• This ruling applies for both Sett and Zhonya's Hourglass
• While the sequential damage doesn't matter for Sett specifically, it does matter for Zhonya's and clarifying timing in general