Rule 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.