Can you cast a split card and only cast one half of it?

Split cards have two separate halves printed on them, but when you cast a split card from your hand (or another zone where you can cast it normally), you choose exactly one half to cast. That half becomes the spell; the other half is ignored for the purpose of that casting. You do not cast both halves at once, and you must cast at least one half — you cannot cast 'none' of the split card.

This is governed by CR 708.2, which states that a player casting a split card chooses which half they are casting before putting it on the stack. Only the chosen half's characteristics (mana cost, type, effects, etc.) apply while the spell is on the stack and resolves.

The only exception is the Fuse mechanic (CR 708.3), found on cards like Breaking // Entering. A card with Fuse allows you to cast both halves simultaneously by paying both mana costs. Without Fuse, you can never cast both halves in a single cast action.

Aftermath cards (CR 708.8) are another variant: the second half can only be cast from the graveyard, so the two halves are intentionally separated into different casting opportunities rather than cast together.

Example: You have Fire // Ice in hand. You may cast Fire (dealing 2 damage) or cast Ice (tapping a creature and drawing a card), but not both from a single casting. You pay the mana cost of whichever half you choose, and only that half's effect resolves.