It shall be unlawful for any person to hold the position of officer or director of more than one carrier subject to this chapter, unless such holding shall have been authorized by order of the Commission, upon due showing in form and manner prescribed by the Commission, that neither public nor private interests will be adversely affected thereby: Provided, That the Commission may authorize persons to hold the position of officer or director in more than one such carrier, without regard to the requirements of this section, where it has found that one of the two or more carriers directly or indirectly owns more than 50 per centum of the stock of the other or others, or that 50 per centum or more of t