If it's somewhere other than the on the stack or if it's fused, a split card has two CMCs. You'll lose the life equal to both of the cards CMCs.
708.5. An effect 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).
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 when this ability resolves, the game sees its converted mana cost as “1, and 4.” You get five creature tokens.