It takes about ten (10) months for the donor area to fully heal.  Therefore if you want another hair transplant you must wait at least ten (10) months.
It takes about one year for all of the transplanted grafts to produce hair.  If we transplant hair onto an area where grafts already exist and where these grafts are not producing hair yet we can damage the grafts by opening channels during the second operation.
You can have your second hair transplant procedure after the final result which will take about ten (10) to twelve (12) months.
For hair that has been transplanted onto the crown area it sometimes takes fifteen (15) months for the final result to be fully realized.