A soul would never choose to split into two.
Split souls may occur when the soul is jolted by outside influences. The energy within the soul begins to retract from itself, causing separation and inner conflict. The combination of separation and inner conflict leads to duality within one soul. This duality is what physically divides twin flames into separate beings.
One part of the soul becomes more physical-based while the other is more spiritual-based. The division of the soul is so devastating, that they each forget that they were once complete. This creates an inner longing for a feeling of completion.
The physical-based one forgets the spiritual and believes that the physical is reality.
The spiritual-based one serves as a guide, in an effort to reach completion. However, the spiritual guide can only be effective if their physical-based counterpart is seeking the spiritual of their own free will.
At times, in order to directly impact the physical, the spiritual guide will incarnate into physical; giving both sides the opportunity to fulfill their mission of regaining a completion.