If something asks for the converted mana cost of a split card that’s not on the stack, that something is going to get two answers instead of one. For instance, if something asks you what the converted mana cost of Boom//Bust is, you’ll get two answers: 2 (for the Boom side) and 6 (for the Bust side). Since it has a converted mana cost of 2, Boom//Bust can be found with Bring to Light as long as you spent at least two different colors of mana to cast it. However, since you’re just instructed to cast the spell, you can cast either half of Boom//Bust, even the side with a converted mana cost of 6! You’re just casting the card, and nothing says you have to cast the side with the lower converted mana cost, so you’ll be able to cast the Bust side of Boom//Bust.
Note that if you search for a card with fuse (like Far//Away), since you’re not casting the split card from your hand (you’re casting it from exile), you won’t be able to fuse the card, even if you spent five different colors of mana to cast Bring to Light: you’ll be stuck casting either Far or Away, not Far//Away fused.
Today’s Rules Tip written by Nathan Long