Can Karthus cause Ekko to trigger his deathknell twice by floating the first set of power to get double value?

Ruling: No, Karthus cannot cause Ekko to trigger his deathknell twice. You must recycle Ekko to ready the runes, and you cannot recycle him twice from a single death.

Nuances:

• The recycle part of Ekko's triggered ability is a condition, not a cost
• Triggered abilities don't have costs; costs are paid when effects enter the chain
• If you fail to recycle Ekko, you don't ready the runes
• Ekko's deathknell can trigger on an opponent's turn whenever he dies