"Joint Managing Director", means "CEO",Chief Executive Officer.
In British English, terms often used as synonyms for CEO are managing director (MD) and chief executive (CE).
"Joint", means "shared, held or done by two or more people together ",
So,Joint Managing Director, means CEO, the chief of a company.
"Deputy Managing Director:, means Vice-MD.
"Deputy", means "person who is immediately below the head of a business, school, etc" that means this position is a vice MD.
Take Amercian Presidents for example,
Barack Hussein Obama is the President of USA, and
Joseph Robinette“Joe”Biden is the Vice-President or Deputy President of USA.
Hope you can understand