second fiddle vs second banana

second fiddle and second banana both are nouns.

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

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

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

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