Breaking

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.

On the stack, a split spell that hasn’t been fused has only that half’s characteristics and mana value. The other half is treated as though it didn’t exist.

To cast a fused split spell, pay both of its mana costs. While the spell is on the stack, its mana value is the total amount of mana in both costs.