One intact 2- hair graft: contain two single hairs follicular unit. This can also be cut into two 1-hair follicular units (paired graft). *paired graft=combine two grafts and insert into one slit.
One intact 3-hair graft: contain one single hair follicular unit and one two-hair follicular unit. This can also be cut into one 1-hair and one 2-hair follicular unit (paired graft).
Conclusion from Dr. Michael Beehner:
Intact follicular unit’s growth rate is better than paired grafts (93% vs. 70%)
3-hair follicular unit has higher growth rate than 2-hair follicular units. (99% vs. 83% on intact 2-hair graft, 49% on paired graft 1 hair+1 hair
Intact follicular units and 3-hair follicular units are our modified follicular units (bi-follicular unit). Paired grafts are single follicular units. This is why our modified follicular unit (bi-follicular unit) has higher growth rate.
Another reason why we opposed to the paired graft is that the goal of hair transplant is to minimize the empty spaces among the grafts. When transplant same size of donor area and hair, paired grafts will increase the empty space.
The photo below shows the scalp skin. If you paired the blue circled hair into adjacent hair, the empty area will increase almost four times.
From another point of view, the intention of hair transplant is to use lesser amount of hair to cover a bigger area. But when you pair grafts, it covers less area. That’s against the purpose of hair transplantation.
Look at the picture above and imaging that if you paired all the hairs, the empty space becomes huge and unnatural. If you want to keep same space then we have to remove 50% of empty skin. Transplant area will be reducing to 50% smaller area.