President Joseph F. Smith in THE IMPROVEMENT ERA, Vol. 10, page 308, answered this question as follows:

"'Does a wife hold the priesthood with her husband, and may she lay hands on the sick with him, with authority?'

"A wife does not hold the priesthood with her husband, but she enjoys the benefits thereof with him; and if she is requested to lay hands on the sick with him, or with any other officer holding the Melchizedek Priesthood, she may do so with perfect propriety. It is no uncommon thing for a man and wife unitedly to administer to their children..."

They would lay hands just as would a member of the Aaronic Priesthood, or a faithful brother without the priesthood, thus giving support by faith to the ordinance. The Prophet Joseph Smith said, "Respecting females administering for the healing of the sick, there could be no evil in it, if God gave his sanction by healing; that there could be no more sin in any female laying hands on and praying for the sick, than in wetting the face with water; it is no sin for anybody to administer that has faith, or if the sick have faith to be healed by their administration." Such an administration would not be by virtue of the priesthood, but a manifestation of faith.