Read the sentence given below:

I have a black and white cow.

The sentence given above means that I have a cow that is partly white and partly black. Now read the following sentence:

I have a black and a white cow. (= I have two cows, one black and the other white.)

Thus we have seen that when two or more adjectives qualify the same noun, the article is used before the first adjective only. When two or more adjectives qualify different nouns, either expressed or understood, the article is used before each adjective.

Compare:

The secretary and treasurer has given his approval to the plan.
The secretary and the treasurer have given their approval to the plan.

Here the first sentence clearly indicates that the posts of the secretary and the treasurer are held by the same person. The repetition of the article in the second sentence indicates that the posts are held by different persons.

More examples are given below:

Winston Churchill was a great orator and statesman. (Here the two nouns orator and statesman refer to the same person – Churchill)
There are on the committee among others a great lawyer and a great writer. (Here the repetition of the article indicates that the nouns lawyer and writer refer to different persons.)

In a comparison, if two nouns refer to the same person or thing, the article is used before the first noun only.

He is a better thinker than writer. (Here both nouns refer to the same person.)
He is a better poet than playwright.

But if the nouns refer to different persons or things, the article is used before each noun.

He would make a better statesman than a philosopher.