Historical Rules · 26 August 2016
Rule 708.2
Official source text from the 26 August 2016 Rules release
In every zone except the stack, split cards have two sets of characteristics and two converted mana costs. As long as a split card is a spell on the stack and wasn’t cast using fuse (see rule 702.101, “Fuse”), only the characteristics of the half being cast exist. The other half’s characteristics are treated as though they didn’t exist. A split card that’s a spell on the stack and was cast using fuse has two sets of characteristics and one converted mana cost. Its converted mana cost is equal to the total amount of mana in its two mana costs, regardless of color.