CA as well as CS

Not possible, because a person can practise either as a CA or as a CS and not both.

 But then, whats the use of doing both? I think there should be something related to the matter.

People do both to acquire knowledge. You can become a member in both the institutes and work as an accountant/ company secretary in a company.

But when it comes to practise, you can choose either of the two and not both.