Split Ray doesn't have a wait-and-see option
The final part of the benefit of feat Split Ray says that "all rays must be aimed at targets within 30 feet of each other and fired simultaneously" (Complete Arcane 83 and emphasis mine).
Thus a caster that casts a ray spell modified by the feat Split Ray picks the targets of his rays then launches all the rays at once.
While the caster can pick to aim both rays at one target then launch both rays at that lone target, the caster can't aim one ray at a target, launch that ray, examine the target to determine if the ray affected that target, then aim the second ray and launch it at the same or a different target. Although that would be awesome, sadly the pair of rays are launched as a group.