If Ekko dies while Karthus is on the bench, can you float all your mana between Ekko triggers?

Ruling: No, you cannot float mana between Ekko triggers because Ekko requires you to recycle to untap runes, so the second trigger will not do anything.

Nuances:

• Ekko's ability to untap runes is dependent on recycling
• The second Ekko trigger becomes ineffective without the recycle action