Historical Rules · 26 August 2016
Rule 708.2b
Official source text from the 26 August 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. 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.