Fire // Ice

If the target permanent becomes an illegal target for Ice, the spell doesn't resolve. You don't draw a card.
If Fire targets two creatures and one becomes an illegal target, the remaining target is dealt 1 damage, not 2.
You divide the damage as you cast Fire, not as it resolves. Each target must be assigned at least 1 damage. In other words, as you cast Fire, you choose whether to have it deal 2 damage to a single target, or deal 1 damage to each of two targets.
All split cards have two card faces on a single card, and you put a split card onto the stack as only the half you're casting. The characteristics of the half of the card you didn't cast are ignored while the spell is on the stack. For example, if an effect says you can't cast blue spells, you may still cast Fire.
If you copy a spell that's half of a split card, the copy copies that same half. For example, if you copy Fire, the copy is also Fire, not Ice.
Each split card has two names. If an effect instructs you to choose a card name, you may choose one of those names, but not both.
While not on the stack, the characteristics of a split card are the combination of its two halves. For example, Fire // Ice is both red and blue and its mana value is 4.
To cast a split card, you choose one half to cast. There's no way to cast both halves of Fire // Ice.
Each split card is a single card. For example, if you discard a split card, you've discarded one card, not two. If an effect counts the number of instant and sorcery cards in your graveyard, Fire // Ice counts once, not twice.