second banana vs straight man

second banana and straight man both are nouns.

Word	Noun	Adjective	Verb	Adverb
second banana	Yes	No	No	No
straight man	Yes	No	No	No

As nouns, second banana and straight man are synonyms defined as:

second banana and straight man: a performer who acts as stooge to a comedian

second banana (noun)	straight man (noun)
a performer who acts as stooge to a comedian	a performer who acts as stooge to a comedian
someone who serves in a subordinate capacity or plays a secondary role