Can Adjective modify another Adjective?

  1. It is a spicy(adj) hot(adj) dish(n).
  2. It is a personal(adj( private(adj) business(n).
  3. It is a bright(adj) red(adj) car(n).
    In the above examples, it looks as if an adjective modifies another adjective.
    Can an adjective modify another adjective?

I think not. But an intensifier can. In your examples the adjectives, though more than one in number, qualify the nouns, viz. dish, business and car.

The dish is spicy; the dish is hot > The dish is spicy and hot (It is a spicy and hot dish).

Can I say it as coordinate adjectives.
“It is a spicy, hot dish.”
The adjectives are separated with comma.

Yes, you may call them so.