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When resolving a fused split spell with multiple targets, treat it as you would any spell with multiple targets. If all targets are illegal when the spell tries to resolve, the spell doesn't resolve and none of its effects happen. If at least one target is still legal at that time, the spell resolves, but an illegal target can't perform any actions or have any actions performed on it.

If you're casting a split card with fuse from any zone other than your hand, you can't cast both halves. You'll only be able to cast one half or the other.

If you cast a split card with fuse from your hand without paying its mana cost, you can choose to use its fuse ability and cast both halves without paying their mana costs.