If I have two recruits and the opponent attacks with a single unit, will Imperial Decree trigger once or twice (does each recruit dealing damage count as a different event)?

Ruling: Imperial Decree will trigger only once. Damage is pooled together and then distributed, so all units take damage in a single event regardless of how many sources are dealing damage.

Nuances:

• If you deal enough damage to kill a unit through Imperial Decree, you can kill it even through Zhonya's. Zhonya's would trigger and resolve first (exhausting, recalling, and healing the unit), but then Imperial Decree would still kill it afterward.
• Having multiple recruits does not create multiple damage events for Imperial Decree purposes.