It is going to be a long, hot summer. She was a faithful, sincere friend.Here's the exception: If the first adjective modifies the idea expressed by the combination of the second adjective and the noun, do not use a comma between the two adjectives:
He favored traditional political institutions.In this example, the expression political institutions is a single idea. Thus, the two adjectives (traditional and political) do not equally modify the single noun. Rather, the first adjective traditional modifies the single unit political institutions.