Identical twins, or even triplets, are incarnate vehicles that share a common Aspect (Soul).
In this instance the True Energetic Self (Higher Self, Over Soul, God Head) elects to spread the energies of the Aspect projected into the physical across two or three incarnate vehicles concurrently. This does not create a shard or a sub-incarnation but the opportunity to let the Aspect experience two or more incarnate existences as if it were one.
Although similar to the creation of shards in the ability to experience multiple incarnations concurrently, it is not the same. This is because the incarnate vehicles actually share one Aspect, which creates a local collective, and not two/three shards from an Aspect, which creates individuality.
As a result of this type of incarnation the twins/triplets can and do (on a higher level) share the same memories, feelings, emotions, thoughts, sensations and actions as a byproduct of the single Aspect occupying these separate incarnate vehicles.