I am a little confused about the usage of a comma in sentences with conjunctions.

For example:

> 
> Dairying is concerned **not only** with the production of milk **but** with the manufacture of milk products.
> 
> 
> 

Can we put a comma before *but*?

Can we say "*Dairying is concerned **not only** with the production of milk, **but** is concerned with the manufacture of milk products.*"? Or must there be an *it* after *but* in this case?

Or is the original sentence wrong at all?