head-to-head vs neck and neck

head-to-head and neck and neck both are adjectives.

Word	Noun	Adjective	Verb	Adverb
head-to-head	No	Yes	No	Yes
neck and neck	No	Yes	No	Yes

As adjectives, head-to-head and neck and neck are synonyms defined as:

head-to-head and neck and neck: inconclusive as to outcome; close or just even in a race or comparison or competition

Other synonyms of head-to-head include nip and tuck.

As adverbs, head-to-head and neck and neck are synonyms defined as:

head-to-head and neck and neck: even or close in a race or competition or comparison

Other synonyms of head-to-head include nip and tuck.

head-to-head (adjective)	neck and neck (adjective)
inconclusive as to outcome; close or just even in a race or comparison or competition	inconclusive as to outcome; close or just even in a race or comparison or competition
involving two persons; intimately private

head-to-head (adverb)	neck and neck (adverb)
even or close in a race or competition or comparison	even or close in a race or competition or comparison