I just read the case details in Ian Stevenson’s book.
Apparently this was a case where a baby, three and a half years old, was on the verge of dying. The Gandhabba of the baby left already, but another Gandhabba of a man who died in a nearby village was advised to “seek shelter” (by another spirit) in the dying body of the 3,5 year old child. The child recovered from the illness and was noticably changed in his personality.
This is only the summary. For more details, the case is well documented.
In this case, it is visible that what happened is a Gandhabba-takeover, not a simultaneous growth and living in two bodies. One might suspect that this is the case for the other cases as well, but I am open for any contrary evidence.