If I have 2 Hourglass cards, do I lose just one or both when I would take lethal damage?

Ruling: You lose only one Hourglass. When you would die, both Hourglass replacement effects trigger and you order them. The first one replaces your death, and the second one has nothing to replace since death was already replaced.

Sequence:

• Both Hourglass replacement effects trigger when you would die
• You choose the order of the replacement effects
• The first Hourglass replacement effect resolves, replacing your death
• The second Hourglass replacement effect cannot replace anything (since death was already replaced by the first effect)
• Only one Hourglass is lost

Nuances:

• Replacement effects don't use the chain
• A replacement effect cannot replace another replacement effect
• The order you choose will matter for future cards like Guardian Angle