I believe you are referring to the practice of taking a naturally occurring multi-hair follicular unit and cutting it into 2 or more smaller grafts. Sometimes this is done to generate more single hair grafts than occur naturally, when large numbers of single hair grafts are needed (e.g., to restore a hairline including the temples). But some clinics that charge by the graft routinely split up grafts in order to artificially inflate the total number and charge more. This usually does a disservice to the patient, since it is generally believed that keeping follicular units intact whenever possible (e.g., preserving a single 3-hair graft as opposed to cutting it into 3 single hair grafts or a single and a double) contributes to better survival and growth. It is important to research and interview your surgeon carefully.
If you are asking about making two grafts out of one, this is done when single hairs are needed for the frontal hairline. It is a difficult process and great care should be taken not to damage the growth centers. We do this under high powered magnification when we do it, but it is rarely done.
is done to generate singles to make a hair line. It can also be done to generate more grafts when the patient doesnt have enough or to simply inflate the numbers in less scrupulous clinics.
Sometimes two or three hair grafts can be split into single hair grafts to be used for front hair line work.