709.4 In every zone except the stack, the characteristics of a split card are those of its two halves combined.
709.4a
Each split card has two names. If an effect instructs a player to choose a card name and the player wants to choose a split card’s name, the player must choose one of those names and not both. An object has the chosen name if one of its names is the chosen name.
709.4b
The mana cost of a split card is the combined mana costs of its two halves. A split card’s colors and mana value are determined from its combined mana cost. An effect that refers specifically to the symbols in a split card’s mana cost sees the separate symbols rather than the whole mana cost. Example: Assault//Battery’s mana cost is {3}{R}{G}. It’s a red and green card with a mana value of 5. If you cast Assault, the resulting spell is a red spell with a mana value of 1. Example: Fire//Ice’s mana cost is {2}{U}{R}. It has the same mana cost as Steam Augury, but an effect such as that of Jegantha, the Wellspring sees that it contains the mana symbol {1} twice.
709.4c
A split card has each card type specified on either of its halves and each ability in the text box of each half.
709.4d
The characteristics of a fused split spell on the stack are also those of its two halves combined (see rule 702.102, “Fuse”).
(refs: 702.102)