second banana vs second fiddle

second banana and second fiddle both are nouns.

Word	Noun	Adjective	Verb	Adverb
second banana	Yes	No	No	No
second fiddle	Yes	No	No	No

As nouns, second banana and second fiddle are synonyms defined as:

second banana and second fiddle: someone who serves in a subordinate capacity or plays a secondary role

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