The Split modifier will unlock two new modifiers for you to add to a Focus. The first will allow you to hit your target with two effects such as Projectile and Bolt. Then, two elements can be added (such as fire and ice). This reduces the damage these effects cause:
The other modifier is Split target. This lets you hit more than one target with two different elemental effects:
As you get further and further into creating more complex Foci, know that a Blank Lesser Focus only allows you to have a max complexity of 15.