straight man vs second banana

straight man and second banana both are nouns.

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

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

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

straight man (noun)	second banana (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