1. These are not superlatives. They are comparatives. 2. The sentence is WRONG. It should be "...more vulnerable OR mighty...." 3. Anyway, yes, when using AND, you MUST put all adjectives in the comparative. The same rule applies to superlatives, by the way. more complicated and louder more intelligent and more beautiful However, with your example you have two opposites. Opposites can be considered as ONE adjective: good and bad; up and down; black and white; yin and yang; east and west In these cases the"more" appears only once. Further east or west. Further up or down. More vulnerable or mighty. More stupid or intelligent.

Firstly you are talking about comparative forms not superlative. Comparative= short one syllable adjectives, adjectives that end in 'y' and some two syllable adjectives take the suffix -er (older) other adjectives use the word 'more'. (more beautiful) Superlative = short one syllable adjectives, adjectives that end in 'y' and some two syllable adjectives take the suffix -est plus 'the' (the greatest) other adjectives take 'the most' (the most beautiful) Normally we don't repeat 'more' when using more than one comparative adjective in a sentence. 'She is more beautiful and intelligent than her sister.' But if the comparative form differs then each adjective has to be in the appropriate form. 'She is older and more beautiful than her sister.' In your example 'more' modifies both 'vulnerable' and 'mighty'. Adjectives that end in 'y' can sometimes form the comparative by using -er or 'more' . More friendly or friendlier. Today we would probably use 'mightier' more than 'more mighty' but in the past 'more mighty' was very common especially in literature. Shakespeare 'tis best to weigh the enemy more mighty than he seems.'

In the case of multiple comparative adjectives ,such as the ones in your example, you will put each consecutive adjective in its unique comparative form. By the way 'more complicated' and 'louder' are comparative forms not superlative forms. The superlative form would be ' the most complicated ' and 'loudest' Each adjective as a rule has the : Comparative form formed usually by adding ER to the adjective or by preceding the adjective by 'more'. Superlative form forms by adding 'est' to the adjective or preceding it by ' the most' Whether you are using the comparative or superlative successively, you will keep each one in its form. In your example: "The song is more complicated and loudER ..." Both adjectives 'complicated' and 'loud' are in their comparative forms. The other sentence should be: There is no creature on Earth more vulnerable and mightier than a lover. The comparative form of 'mighty' is 'mightier' not 'more mighty'.