BA.LLB with CS(company secretary) legally valid or not ?

It is valid, indeed. And there's said to be quite a lot of scope plus it is considered convenient as well as a large chunk of BA.LLB forms a signifcant part of CS as well. Just ensure you can manage the time well.

Is it legally vaid to do CS along with BA.LLB ?

Personal Opinion 

Everyone has made education system a big mess, where no one knows what is right and what is wrong and this is the truth.  Nowadays trend has started student take admission in classes before 11std , attendance is faked in college (officially) prepare for some CET etc. and take admission in the proffessional course.  Everyone knows the truth. And called this system with one or other name. 

If you can do both courses in your capacity then do it . At least be happy you are giving extra efforts for study than wasting time outside watching some movie.   Good Luck 

And forget that education rules. (If you do little research one professional body formed by one act of parliament is in dispute with other body formed by another act of parliament then another university then other colleges, then other state Govt.  All are in dispute in Hon. Supreme Court and at the end whose degree will be valid God knows? Who is correct? This is situation of all professional courses in India (Engineering, Architecture, medical, etc)  )