even chance vs toss-up

even chance and toss-up both are nouns.

Word	Noun	Adjective	Verb	Adverb
even chance	Yes	No	No	No
toss-up	Yes	No	No	No

As nouns, even chance and toss-up are synonyms defined as:

Other synonyms of even chance include tossup.

even chance (noun)	toss-up (noun)
an unpredictable phenomenon	an unpredictable phenomenon