The consent of all the co-owners shall be necessary in voting shares of stock owned jointly by two (2) or more persons, unless there is a written proxy, signed by all the co-owners, authorizing one (1) or some of them or any other person to vote such share or shares: Provided, That when the shares are owned in an “and/or” capacity by the holders thereof, any one of the joint owners can vote said shares or appoint a proxy therefor.