Historical Rules · 30 September 2016
Rule 708.2b
Official source text from the 30 September 2016 Rules release
If a player casts a split card with fuse from his or her hand, in addition to choosing either half as described above, the player may choose to cast both halves, resulting in a fused split spell with the combined characteristics of both halves. Only the fused split spell is evaluated to see if it can be cast.