What is the converted mana cost of fuse cards

Question

If I were to play Blast of Genius and chose to discard Catch // Release Would the converted mana cost be the total cost of both halves or one?

Answer 1

Its converted mana cost is 9. Blast of Genius will deal 9 damage. Rule 708.4 says

• 708.4. In every zone except the stack, the characteristics of a split card are those of its two halves combined. This is a change from previous rules.

  • 708.4b The mana cost of a split card is the combined mana costs of its two halves. A split card’s colors and converted mana cost are determined from its combined mana cost.

Catch has a mana cost of {1}{U}{R} and Release has a mana cost of {4}{R}{W}, so the combined mana cost is {5}{W}{U}{R}{R} and the converted mana cost is 9.

Answer 2

It has two converted mana costs, 3 and 6. It will deal 3 damage and 6 damage simultaneously, which is the same as doing 9 damage.

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Outside of the stack, Catch // Release has two CMCs.

708.2. In every zone except the stack, split cards have two sets of characteristics and two converted mana costs. [...]

708.5. Anything that asks for a particular characteristic of a split card while it’s in a zone other than the stack gets two answers (one for each of the split card’s two halves). [...]

So Catch // Release had CMCs 3 and 6 when it was discarded..

Since Blast of Genius doesn't compare CMC but gets its value, you perform the action using the CMC for each CMC the card has.

708.5's Example: Infernal Genesis has an ability that reads, “At the beginning of each player’s upkeep, that player puts the top card from his or her library into his or her graveyard. He or she then puts X 1/1 black Minion creature tokens onto the battlefield, where X is that card’s converted mana cost.” If the top card of your library is Assault/Battery, which has converted mana costs of 1 and 4, when this ability resolves, you simultaneously put one and four creature tokens onto the battlefield, for a total of five.

A ruling on Blast of Genius confirms this:

If you discard a split card, Blast of Genius will deal damage equal the sum of its two converted mana costs as a single damage-dealing event.