Dragon's Maze brought us split cards again, this time, however, with a special ability. Before we set to discuss this absolutely new unprecedented mechanics — Fuse, let us refresh our knowledge on the split cards by taking a close look at one of them.
Everything described above concerns split cards outside the stack exclusively. Now let us discuss the split cards on the stack.
(708.2.) 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.
The above shows that fused split cards are treated by the game in a special way.
708.2a. If a player casts a split card without using fuse, that player chooses which half of that split card he or she is casting before putting it onto the stack. Only that half is evaluated to see if it can be cast. Only that half is considered to be put onto the stack.
According to the rules, it is considered that only one half is on the stack, the one that's being played. The other half is ignored by the game at this time. When copying a spell, for example with Twincast, we only get the half that's on the stack. Note that the effect of Isochron Scepter copies a card, not a spell, which allows us to play any half of the imprinted card.
if upon resolving Ice its target is illegal, Ice will be countered and the player doesn't draw a card!
708.7. Each split card has two names. If an effect instructs a player to name a card and the player wants to name a split card, the player must name one of those names and not both. An object has the chosen name if one of its names is the chosen name.
It's a new rule. It changes the way cards like Meddling Mage work related to split cards. For instance, if the cunning opponent calls Fire, you will be able to play Ice (you couldn't do it with old rules). If the opponent calls Alive, you will be able to play Well, but you won't be able to fuse Alive/Well to play both, since such a spell has two names, one of which was called for the forbidding effect:
708.2b. If a player casts a split card with fuse from his or her hand, in addition to choosing either half as described above (in 708.2a), the player may choose to cast both halves, resulting in a fused split spell. Both halves are evaluated to see if the spell can be cast. If either half can't be cast, then both halves can't be cast as a single spell.